Update minimum macOS to 10.7.

Author: dvander

AMBuildScript

@@ -317,9 +317,9 @@ class AMXXConfig(object):
 
   def configure_mac(self, cxx):
     cxx.defines += ['OSX', '_OSX', 'POSIX']
-    cxx.cflags += ['-mmacosx-version-min=10.5']
+    cxx.cflags += ['-mmacosx-version-min=10.7']
     cxx.linkflags += [
-      '-mmacosx-version-min=10.5',
+      '-mmacosx-version-min=10.7',
       '-arch', 'i386',
       '-lstdc++',
       '-stdlib=libc++',