contrib: MacOSX 10.5 works fine (tested for PPC32)

Use this as the default
Specify which framework to use by using OSX_VERSION=10.6

contrib/src/main.mak

@@ -90,7 +90,7 @@ endif
 endif
 
 ifdef HAVE_MACOSX
-OSX_VERSION=10.6
+OSX_VERSION?=10.5
 MACOSX_SDK=/Developer/SDKs/MacOSX$(OSX_VERSION).sdk
 CC=gcc-4.2
 CXX=g++-4.2