Explicitly ignore potential unwanted pkg-config found on Mac OS X

We want to be buildable on Mac OS X without pkg-config. For instance
Mono installs a symlink /usr/bin/pkg-config that just confuses this
goal if you happen to build LibreOffice on a machine with Mono
installed.

configure.in

@@ -2416,7 +2416,7 @@ if test "$_os" != "WINNT" -o "$WITH_MINGW" = "yes"; then
    AC_CHECK_TOOL(AR,ar)
    AC_CHECK_TOOL(NM,nm)
    AC_CHECK_TOOL(OBJDUMP,objdump)
-   AC_CHECK_TOOL(PKG_CONFIG,pkg-config)
+   AC_PATH_TOOL(PKG_CONFIG,pkg-config)
    AC_CHECK_TOOL(RANLIB,ranlib)
    AC_CHECK_TOOL(STRIP,strip)
    if test "$_os" = "WINNT"; then
@@ -2433,6 +2433,25 @@ AC_SUBST(RANLIB)
 AC_SUBST(STRIP)
 AC_SUBST(WINDRES)
 
+dnl ===================================================================
+dnl pkg-config checks on Mac OS X
+dnl ===================================================================
+
+if test $_os = Darwin; then
+   AC_MSG_CHECKING([for bogus pkg-config])
+   if test -n "$PKG_CONFIG"; then
+      if test "$PKG_CONFIG" = /usr/bin/pkg-config && ls -l /usr/bin/pkg-config | grep -q Mono.framework; then
+         AC_MSG_RESULT([yes, from Mono, ignoring])
+         PKG_CONFIG=""
+      else
+         AC_MSG_RESULT([yes, from unknown origin, ignoring])
+         PKG_CONFIG=""
+      fi
+   else
+      AC_MSG_RESULT([no])
+   fi
+fi
+
 dnl ===================================================================
 dnl  .NET needs special treatment
 dnl (does the above comment really mean .NET, or is it misusing