Bug 286980 - cairo config should add to PKG_CONFIG_PATH, not replace. r=cls

configure.in

@@ -6469,7 +6469,7 @@ if test "$MOZ_TREE_CAIRO"; then
     AC_OUTPUT_SUBDIRS(gfx/cairo/libpixman)
 
     PKG_CONFIG_PATH=$MOZ_BUILD_ROOT/gfx/cairo/libpixman:$PKG_CONFIG_PATH
-    export $PKG_CONFIG_PATH
+    export PKG_CONFIG_PATH
     ac_configure_args="$_SUBDIR_CONFIG_ARGS --prefix=$MOZ_BUILD_ROOT/dist --disable-ps --disable-pdf --disable-png --disable-glitz --disable-xcb"
     if test -n "$MACOSX"; then
         ac_configure_args="$ac_configure_args --disable-freetype --disable-xlib"