Fix build for OS X (CMake)

(a cherry-pick of commit f73611c from 'master')

Issue #275 (bdwgc).

Having "ppc" in CMAKE_OSX_ARCHITECTURES fails the build on Mac
OS X 10.13, at least.

* CMakeLists.txt [APPLE && $CMAKE_OSX_ARCHITECTURES==""]
(CMAKE_OSX_ARCHITECTURES): Set to "x86_64;i386" (instead of
"ppc;i386;x86_64").

CMakeLists.txt

@@ -34,7 +34,8 @@ ADD_DEFINITIONS("-D_CRT_SECURE_NO_DEPRECATE
 
 IF(APPLE)
     IF("${CMAKE_OSX_ARCHITECTURES}" STREQUAL "")
-        SET(CMAKE_OSX_ARCHITECTURES "ppc;i386;x86_64" CACHE STRING "Build architectures for Mac OS X" FORCE)
+        SET(CMAKE_OSX_ARCHITECTURES "x86_64;i386"
+            CACHE STRING "Build architectures for Mac OS X" FORCE)
     ENDIF()
 ENDIF(APPLE)