Reduce some unnecessary text duplication in the Makefile.

Instead of duplicating the value of 'GCC_DEP' in every package's deps_
variable, just set it once as a variabele in the Makefile itself, and then
include it in the output of the pkg_deps helper function.

build/make/Makefile.in

@@ -39,6 +39,11 @@ BLAS = @SAGE_BLAS@
 BUILT_PACKAGES = @SAGE_BUILT_PACKAGES@
 DUMMY_PACKAGES = @SAGE_DUMMY_PACKAGES@
 
+# Set to the path to Sage's GCC (if GCC is installed) to force rebuilds
+# of packages if GCC changed.
+# See m4/sage_spkg_collect.m4 and https://trac.sagemath.org/ticket/24703
+GCC_DEP = @SAGE_GCC_DEP@
+
 # Versions of all the packages, in the format
 #
 # vers_<pkgname> = <pkgvers>
@@ -128,9 +133,13 @@ $(INST)/.dummy:
 # the name of some arbitrary file, or it is the '|' symbol) then it is just
 # used verbatim.
 #
+# As a special case, also adds a special variable GCC_DEP for all packages
+# except for gcc itself.  See the definition of GCC_DEP above
+#
 # Positional arguments:
 #     $(1): package name
 pkg_deps = \
+	$(if $(filter gcc,$(1)),,$$(GCC_DEP))\
 	$(foreach dep,$(deps_$(1)),\
         $(if $(value inst_$(dep)),$$(inst_$(dep)),$(dep)))
 

m4/sage_spkg_collect.m4

@@ -57,10 +57,11 @@ AC_DEFUN_ONCE([SAGE_SPKG_COLLECT], [
 # (except gcc) should depend on gcc if gcc is already installed.
 # See https://trac.sagemath.org/ticket/24703
 if test x$SAGE_INSTALL_GCC = xexists; then
-    GCC_DEP='$(SAGE_LOCAL)/bin/gcc'
+    SAGE_GCC_DEP='$(SAGE_LOCAL)/bin/gcc'
 else
-    GCC_DEP=''
+    SAGE_GCC_DEP=''
 fi
+AC_SUBST([SAGE_GCC_DEP])
 
 AC_MSG_CHECKING([package versions])
 AC_MSG_RESULT([])
@@ -192,9 +193,6 @@ for DIR in $SAGE_ROOT/build/pkgs/*; do
         esac
     fi
 
-    # Special case for GCC; see definition of GCC_DEP above
-    test "$PKG_NAME" = gcc || DEPS="$GCC_DEP $DEPS"
-
     SAGE_PACKAGE_DEPENDENCIES+="deps_$PKG_NAME = $DEPS"$'\n'
 
     # Determine package build rules