Remove gettext download for Mac in CMake build

And added a python version that will be used on all platforms
when a suitable msgfmt isn't installed.  (As long as python
is installed.)

Extracted all the Audacity specific functions from main cmake
list and moved them to their own module.

Rearrange the main cmake module a bit and misc. cleanup.

src/CMakeLists.txt

@@ -1015,6 +1015,10 @@ list( APPEND DEFINES
       WXINTL_NO_GETTEXT_MACRO
       WXUSINGDLL
       CMAKE
+      $<$<BOOL:${GIT_FOUND}>:
+         REV_LONG="${GIT_COMMIT_LONG}"
+         REV_TIME="${GIT_COMMIT_TIME}"
+      >
       $<$<BOOL:${HAVE_LRINT}>:
          HAVE_LRINT
       >