Make build process set PKG_CONFIG_PATH variable

If the platform has pkg-config then add python's .pc files to the end of
the pkg-config search path. If there was a problem setting it or
pkg-config doesn't exist then do nothing.

It appears that the environment variable change only exists within the
build process.

setupext.py

@@ -258,6 +258,20 @@ def get_win32_compiler():
 else:
     std_libs = ['stdc++', 'm']
 
+def set_pkgconfig_path():
+    pkgconfig_path = sysconfig.get_config_var('LIBDIR')
+    if pkgconfig_path is None:
+        return
+
+    pkgconfig_path += '/pkgconfig'
+    if not os.path.isdir(pkgconfig_path):
+        return
+
+    try:
+        os.environ['PKG_CONFIG_PATH'] += ':' + pkgconfig_path
+    except KeyError:
+        os.environ['PKG_CONFIG_PATH'] = pkgconfig_path
+
 def has_pkgconfig():
     if has_pkgconfig.cache is not None:
         return has_pkgconfig.cache
@@ -267,6 +281,10 @@ def has_pkgconfig():
         #print 'environ',  os.environ['PKG_CONFIG_PATH']
         status, output = getstatusoutput("pkg-config --help")
         has_pkgconfig.cache = (status == 0)
+
+        # Set the PKG_CONFIG_PATH environment variable
+        if has_pkgconfig.cache:
+            set_pkgconfig_path()
     return has_pkgconfig.cache
 has_pkgconfig.cache = None