Bugfix: the user provided path needs to be _pre_pended to PKG_CONFIG_PATH

Otherwise a system-wide installation will always override the
user choice.

[[Imported from SVN: r7627]]

m4/ipopt.m4

@@ -31,7 +31,7 @@ AS_IF([test x$with_ipopt != xno], [
     IPOPT_LIB_PATH="$IPOPTROOT/lib"
 
     # Check for ipopt and set IPOPT_*
-    export PKG_CONFIG_PATH="$PKG_CONFIG_PATH:$IPOPT_LIB_PATH/pkgconfig"
+    export PKG_CONFIG_PATH="$IPOPT_LIB_PATH/pkgconfig:$PKG_CONFIG_PATH"
     PKG_CHECK_MODULES([IPOPT], [ipopt], [
 	HAVE_IPOPT="1"
     ], [ dnl default rule would fail