change the CFLAGS controlled by menuselect to be placed in a header file

change the CFLAGS controlled by menuselect to be placed in a header file
instead of being added to the compiler commands.  This header file will be
installed and modules built outside of the main tree will be able to use the
same build options used to build the rest of Asterisk.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@26808 65c4cc65-6c06-0410-ace0-fbb531ad65f3