Prefer vendored qhull if sys-wide version can't be determined.

1. When qhull is available system-wide but its version cannot be
determined, ignore it (there is no guarantee that it will, in fact,
work) and fallback to the vendored version.
2. When adding header paths to vendored libraries, prepend the paths to
give them higher priority than /usr/{,local/}include.  (In reality we
should just not add the system /usr/{,local/}include paths to the
compiler invocations at all, but that's a more complicated patch.

setupext.py

@@ -1033,7 +1033,7 @@ def add_flags(self, ext, add_sources=True):
         if self.found_external:
             pkg_config.setup_extension(ext, 'libagg')
         else:
-            ext.include_dirs.append('extern/agg24-svn/include')
+            ext.include_dirs.insert(0, 'extern/agg24-svn/include')
             if add_sources:
                 agg_sources = [
                     'agg_bezier_arc.cpp',
@@ -1348,23 +1348,15 @@ def check(self):
                 'libqhull', 'libqhull/qhull_a.h', min_version='2015.2')
         except CheckFailed as e:
             self.__class__.found_pkgconfig = False
-            # Qhull may not be in the pkg-config system but may still be
-            # present on this system, so check if the header files can be
-            # found.
-            include_dirs = [
-                os.path.join(x, 'libqhull') for x in get_include_dirs()]
-            if has_include_file(include_dirs, 'qhull_a.h'):
-                return 'Using system Qhull (version unknown, no pkg-config info)'
-            else:
-                self.__class__.found_external = False
-                return str(e) + ' Using local copy.'
+            self.__class__.found_external = False
+            return str(e) + ' Using local copy.'
 
     def add_flags(self, ext):
         if self.found_external:
             pkg_config.setup_extension(ext, 'qhull',
                                        default_libraries=['qhull'])
         else:
-            ext.include_dirs.append('extern')
+            ext.include_dirs.insert(0, 'extern')
             ext.sources.extend(sorted(glob.glob('extern/libqhull/*.c')))
 
 
@@ -1380,7 +1372,7 @@ def get_extension(self):
             ]
         ext = make_extension('matplotlib.ttconv', sources)
         Numpy().add_flags(ext)
-        ext.include_dirs.append('extern')
+        ext.include_dirs.insert(0, 'extern')
         return ext
 
 
@@ -1536,7 +1528,7 @@ def get_extension(self):
         return ext
 
     def add_flags(self, ext):
-        ext.include_dirs.extend(['src'])
+        ext.include_dirs.insert(0, 'src')
         if sys.platform == 'win32':
             # PSAPI library needed for finding Tcl / Tk at run time
             ext.libraries.extend(['psapi'])