make the new memory allocator...

...(or whatever else caused the game to freeze at showing the
mainscreen) work with cmake, too

in all other build systems this define was set due to sdl-config,
might require an explicit addition, just to be sure

CMakeLists.txt

@@ -107,6 +107,9 @@ if(X11_FOUND)
   add_definitions(-D_X11)
 endif(X11_FOUND)
 
+# needed to have the new memory allocator work!!!
+add_definitions(-D_GNU_SOURCE=1)
+
 add_definitions(-DHAS_RELATIVE_LOCALEDIR)
 add_definitions(-DLOCALEDIR='\"${LOCALEDIR}\"')
 set(LOCALE_INSTALL ${DATADIR}/${LOCALEDIR})