Fix test cases/common/37 on Cygwin

Including newlib's <stdlib.h> brings in a '#define __has_include 0', so
using -U__has_include on the command line isn't going to remove it (so the
fallback doesn't happen and the test fails)

Instead use a '#undef __has_include' at the end of the prefix to excerise
this.

(newlib's <stdlib.h> is derived from FreeBSD, so the same problem will
probably be seen there)

test cases/common/37 has header/meson.build

@@ -11,19 +11,19 @@ configure_file(input : non_existant_header,
 
 # Test that the fallback to __has_include also works on all compilers
 if host_system != 'darwin'
-  args = [[], ['-U__has_include']]
+  fallbacks = ['', '\n#undef __has_include']
 else
   # On Darwin's clang you can't redefine builtin macros so the above doesn't work
-  args = [[]]
+  fallbacks = ['']
 endif
 
-foreach arg : args
+foreach fallback : fallbacks
   foreach comp : [meson.get_compiler('c'), meson.get_compiler('cpp')]
-    assert(comp.has_header('stdio.h', args : arg), 'Stdio missing.')
+    assert(comp.has_header('stdio.h', prefix : fallback), 'Stdio missing.')
 
     # stdio.h doesn't actually need stdlib.h, but just test that setting the
     # prefix does not result in an error.
-    assert(comp.has_header('stdio.h', prefix : '#include <stdlib.h>', args : arg),
+    assert(comp.has_header('stdio.h', prefix : '#include <stdlib.h>' + fallback),
            'Stdio missing.')
 
     # XInput.h should not require type definitions from windows.h, but it does
@@ -32,23 +32,23 @@ foreach arg : args
     # We only do this check on MSVC because MinGW often defines its own wrappers
     # that pre-include windows.h
     if comp.get_id() == 'msvc'
-      assert(comp.has_header('XInput.h', prefix : '#include <windows.h>', args : arg),
+      assert(comp.has_header('XInput.h', prefix : '#include <windows.h>' + fallback),
              'XInput.h should not be missing on Windows')
-      assert(comp.has_header('XInput.h', prefix : '#define _X86_', args : arg),
+      assert(comp.has_header('XInput.h', prefix : '#define _X86_' + fallback),
              'XInput.h should not need windows.h')
     endif
 
     # Test that the following GCC bug doesn't happen:
     # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80005
     # https://github.com/mesonbuild/meson/issues/1458
     if host_system == 'linux'
-      assert(comp.has_header('linux/if.h', args : arg),
+      assert(comp.has_header('linux/if.h', prefix : fallback),
              'Could not find <linux/if.h>')
     endif
 
     # This header exists in the source and the builddir, but we still must not
     # find it since we are looking in the system directories.
-    assert(not comp.has_header(non_existant_header, args : arg),
+    assert(not comp.has_header(non_existant_header, prefix : fallback),
            'Found non-existant header.')
   endforeach
 endforeach