diff --git a/Library/Homebrew/extend/ENV.rb b/Library/Homebrew/extend/ENV.rb
index b1d7eab120..e8c0c0ad3a 100644
--- a/Library/Homebrew/extend/ENV.rb
+++ b/Library/Homebrew/extend/ENV.rb
@@ -14,12 +14,12 @@ module HomebrewEnvExtension
       # it encounters files with mixed character sets
       delete('LC_ALL')
       self['LC_CTYPE']="C"
-
-      # Mountain Lion no longer ships a few .pcs; make sure we pick up our versions
-      prepend 'PKG_CONFIG_PATH',
-        HOMEBREW_REPOSITORY/'Library/Homebrew/pkgconfig', ':'
     end
 
+    # Set the default pkg-config search path, overriding the built-in paths
+    # Anything in PKG_CONFIG_PATH is searched before paths in this variable
+    self['PKG_CONFIG_LIBDIR'] = determine_pkg_config_libdir
+
     # make any aclocal stuff installed in Homebrew available
     self['ACLOCAL_PATH'] = "#{HOMEBREW_PREFIX}/share/aclocal" if MacOS::Xcode.provides_autotools?
 
@@ -66,6 +66,15 @@ module HomebrewEnvExtension
     end
   end
 
+  def determine_pkg_config_libdir
+    paths = []
+    paths << HOMEBREW_PREFIX/'lib/pkgconfig'
+    paths << HOMEBREW_PREFIX/'share/pkgconfig'
+    paths << HOMEBREW_REPOSITORY/'Library/Homebrew/pkgconfig' if MacOS.version >= :mountain_lion
+    paths << '/usr/lib/pkgconfig'
+    paths.select { |d| File.directory? d }.join(':')
+  end
+
   def deparallelize
     remove 'MAKEFLAGS', /-j\d+/
   end
@@ -249,10 +258,13 @@ module HomebrewEnvExtension
 
   def x11
     # There are some config scripts here that should go in the PATH
-    prepend 'PATH', MacOS::X11.bin, ':'
+    append 'PATH', MacOS::X11.bin, ':'
 
-    prepend 'PKG_CONFIG_PATH', MacOS::X11.lib/'pkgconfig', ':'
-    prepend 'PKG_CONFIG_PATH', MacOS::X11.share/'pkgconfig', ':'
+    # Append these to PKG_CONFIG_LIBDIR so they are searched
+    # *after* our own pkgconfig directories, as we dupe some of the
+    # libs in XQuartz.
+    append 'PKG_CONFIG_LIBDIR', MacOS::X11.lib/'pkgconfig', ':'
+    append 'PKG_CONFIG_LIBDIR', MacOS::X11.share/'pkgconfig', ':'
 
     append 'LDFLAGS', "-L#{MacOS::X11.lib}"
     append 'CMAKE_PREFIX_PATH', MacOS::X11.prefix, ':'