cmake: use pkg-config for mng detection

Signed-off-by: Alexander Golubev <fatzer2@gmail.com>

ConfigureChecks.cmake

@@ -62,17 +62,13 @@ endif( WITH_JPEG )
 ##### check for libmng
 
 if( WITH_LIBMNG )
-    find_path( LIBMNG_HEADER libmng.h )
-    find_library( LIBMNG_LIBRARY mng )
-    if( LIBMNG_HEADER AND LIBMNG_LIBRARY )
-        set( LIB_MNG_INCLUDE_DIRS "${LIBMNG_HEADER}" )
-        set( LIB_MNG_LIBRARIES "${LIBMNG_LIBRARY}" )
-     else()
-        tde_message_fatal( "Libmng support has been requested but was not found on your system" )
-     endif()
-     if( (NOT WITH_JPEG) OR (NOT JPEG_FOUND) )
+    if( NOT WITH_JPEG )
           tde_message_fatal( "libmng support requires jpeg support too" )
     endif()
+    pkg_search_module( LIB_MNG libmng )
+    if( NOT LIB_MNG_FOUND )
+        tde_message_fatal( "Libmng support has been requested but was not found on your system" )
+     endif()
  else()
     set( NO_IMAGEIO_MNG TQT_NO_IMAGEIO_MNG )
 endif( WITH_LIBMNG )