Fix for test cases/frameworks/17 mpi on Cygwin

Unusually, the cygwin package for openmpi installs headers into
/usr/include, rather than /usr/include/openmpi

Prevent pkg-config from suppressing that -I flag, so a gfortran INCLUDE can
find those files

mesonbuild/dependencies/base.py

@@ -495,7 +495,13 @@ class PkgConfigDependency(ExternalDependency):
         return converted
 
     def _set_cargs(self):
-        ret, out = self._call_pkgbin(['--cflags', self.name])
+        env = None
+        if self.language == 'fortran':
+            # gfortran doesn't appear to look in system paths for INCLUDE files,
+            # so don't allow pkg-config to suppress -I flags for system paths
+            env = os.environ.copy()
+            env['PKG_CONFIG_ALLOW_SYSTEM_CFLAGS'] = '1'
+        ret, out = self._call_pkgbin(['--cflags', self.name], env=env)
         if ret != 0:
             raise DependencyException('Could not generate cargs for %s:\n\n%s' %
                                       (self.name, out))