Moved generic compiler options to AM_CFLAGS.  MPD_CFLAGS/MPD_LIBS will
hopefully fade away one day, in favor of more fine-grained variables.

Previously, it was not possible to check for e.g. "-std=c99".

needed for compatibility with older gcc.  Also, threw in
some warnings for things I find offensive
(declaration-after-statement, shadow)

git-svn-id: https://svn.musicpd.org/mpd/trunk@7299 09075e82-0dd4-0310-85a5-a0d7c8717e4f