Allow linking against a system qhull as well.

INSTALL.rst

@@ -109,7 +109,6 @@ Dependencies
 Matplotlib requires the following dependencies:
 
 * `Python <https://www.python.org/downloads/>`_ (>= 3.6)
-* `FreeType <https://www.freetype.org/>`_ (>= 2.3)
 * `NumPy <http://www.numpy.org>`_ (>= 1.11)
 * `setuptools <https://setuptools.readthedocs.io/en/latest/>`_
 * `cycler <http://matplotlib.org/cycler/>`_ (>= 0.10.0)
@@ -154,24 +153,27 @@ etc., you can install the following:
 * `LaTeX <https://miktex.org/>`_ and `GhostScript (>=9.0)
   <https://ghostscript.com/download/>`_ : for rendering text with LaTeX.
 
-FreeType
---------
+FreeType and Qhull
+------------------
 
-Matplotlib depends on FreeType, a font rendering library.  By default,
-Matplotlib downloads and builds its own copy of FreeType.
+Matplotlib depends on `FreeType <https://www.freetype.org/>`_ (>= 2.3), a
+font rendering library, and on `Qhull <http://www.qhull.org/>`_ (>= 2015.2),
+a library for computing triangulations.  By default, Matplotlib downloads and
+builds its own copy of FreeType, and uses its own copy of Qhull.
 
-To force Matplotlib to use a copy of FreeType already installed in your system,
-create a :file:`setup.cfg` file with the following contents:
+To force Matplotlib to use a copy of FreeType or Qhull already installed in
+your system, create a :file:`setup.cfg` file with the following contents:
 
 .. code-block:: cfg
 
    [libs]
    system_freetype = true
+   system_qhull = true
 
 before running ``python -m pip install .``.
 
-In this case, you need to install the FreeType library and headers.  This can
-be achieved using a package manager:
+In this case, you need to install the FreeType and Qhull library and headers.
+This can be achieved using a package manager, e.g. for FreeType:
 
 .. code-block:: sh
 
@@ -181,6 +183,8 @@ be achieved using a package manager:
    brew install freetype  # macOS with Homebrew
    conda install freetype  # conda, any OS
 
+(adapt accordingly for Qhull).
+
 On Linux and macOS, it is also recommended to install pkg-config_, a helper
 tool for locating FreeType:
 
@@ -197,7 +201,7 @@ tool for locating FreeType:
 .. _pkg-config: https://www.freedesktop.org/wiki/Software/pkg-config/
 
 If not using pkg-config (in particular on Windows), you may need to set the
-include path (to the FreeType headers) and link path (to the FreeType library)
+include path (to the library headers) and link path (to the libraries)
 explicitly, if they are not in standard locations.  This can be done using
 standard environment variables -- on Linux and OSX:
 
@@ -215,10 +219,9 @@ and on Windows:
 
 .. note::
 
-  The following libraries are shipped with Matplotlib:
+  Matplotlib always uses its own copies of the following libraries:
 
   - ``Agg``: the Anti-Grain Geometry C++ rendering engine;
-  - ``qhull``: to compute Delaunay triangulation;
   - ``ttconv``: a TrueType font utility.
 
 Building on Windows

setup.cfg.template

@@ -4,10 +4,11 @@
 [egg_info]
 
 [libs]
-# By default, Matplotlib downloads and builds its own copy of FreeType.  You
-# may set the following variable to True to instead link against a system
-# FreeType.
+# By default, Matplotlib downloads and builds its own copy of FreeType, and
+# builds its own copy of Qhull.  You may set the following to True to instead
+# link against a system FreeType/Qhull.
 #system_freetype = False
+#system_qhull = False
 
 [packages]
 # There are a number of data subpackages from Matplotlib that are

setupext.py

@@ -149,20 +149,17 @@ def write_cache(local_fn, data):
 
 
 # matplotlib build options, which can be altered using setup.cfg
-options = {
-    'backend': None,
-}
-
-
 setup_cfg = os.environ.get('MPLSETUPCFG', 'setup.cfg')
+config = configparser.ConfigParser()
 if os.path.exists(setup_cfg):
-    config = configparser.ConfigParser()
     config.read(setup_cfg)
-    options['backend'] = config.get('rc_options', 'backend', fallback=None)
-    options['system_freetype'] = config.getboolean('libs', 'system_freetype',
-                                                   fallback=False)
-else:
-    config = None
+options = {
+    'backend': config.get('rc_options', 'backend', fallback=None),
+    'system_freetype': config.getboolean('libs', 'system_freetype',
+                                         fallback=False),
+    'system_qhull': config.getboolean('libs', 'system_qhull',
+                                      fallback=False),
+}
 
 
 if '-q' in sys.argv or '--quiet' in sys.argv:
@@ -318,8 +315,7 @@ def get_config(cls):
         insensitively defined as 0, false, no, off for False).
         """
         conf = cls.default_config
-        if (config is not None
-                and config.has_option(cls.config_category, cls.name)):
+        if config.has_option(cls.config_category, cls.name):
             try:
                 conf = config.getboolean(cls.config_category, cls.name)
             except ValueError:
@@ -614,13 +610,13 @@ class Qhull(SetupPackage):
     name = "qhull"
 
     def add_flags(self, ext):
-        # Qhull doesn't distribute pkg-config info, so we have no way of
-        # knowing whether a system install is recent enough.  Thus, always use
-        # the vendored version.
-        ext.include_dirs.insert(0, 'extern')
-        ext.sources.extend(sorted(glob.glob('extern/libqhull/*.c')))
-        if sysconfig.get_config_var('LIBM') == '-lm':
-            ext.libraries.extend('m')
+        if options.get('system_qhull'):
+            ext.libraries.append('qhull')
+        else:
+            ext.include_dirs.insert(0, 'extern')
+            ext.sources.extend(sorted(glob.glob('extern/libqhull/*.c')))
+            if sysconfig.get_config_var('LIBM') == '-lm':
+                ext.libraries.extend('m')
 
 
 class TTConv(SetupPackage):