OS X: Use 'uname -m' for processor (#14712)

In commit 2412d9bc (Use 'uname -m' for processor on Cygwin, 2010-09-21)
it was pointed out that POSIX requires 'uname -m' but not 'uname -p':

  http://www.opengroup.org/onlinepubs/9699919799/utilities/uname.html

On OS X x86_64, 'uname -p' returns i386 while 'uname -m' returns x86_64.
Since the latter is the POSIX-compliant option trust its value.
However, on OS X ppc, 'uname -m' returns 'Power Macintosh' so add a
special-case to convert that to 'powerpc'.

Modules/CMakeDetermineSystem.cmake

@@ -47,9 +47,14 @@ if(CMAKE_HOST_UNIX)
   if(CMAKE_UNAME)
     exec_program(uname ARGS -s OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_NAME)
     exec_program(uname ARGS -r OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_VERSION)
-    if(CMAKE_HOST_SYSTEM_NAME MATCHES "Linux|CYGWIN.*")
+    if(CMAKE_HOST_SYSTEM_NAME MATCHES "Linux|CYGWIN.*|Darwin")
       exec_program(uname ARGS -m OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_PROCESSOR
         RETURN_VALUE val)
+      if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin" AND
+         CMAKE_HOST_SYSTEM_PROCESSOR STREQUAL "Power Macintosh")
+        # OS X ppc 'uname -m' may report 'Power Macintosh' instead of 'powerpc'
+        set(CMAKE_HOST_SYSTEM_PROCESSOR "powerpc")
+      endif()
     elseif(CMAKE_HOST_SYSTEM_NAME MATCHES "OpenBSD")
       exec_program(arch ARGS -s OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_PROCESSOR
         RETURN_VALUE val)