Since we use bashisms in build_tools/mkpkgconfig, we should call on bash
explicitly when running from the Makefile, otherwise we get errors during a
'make install.'

(closes issue #15209)
Reported by: seandarcy


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@197260 65c4cc65-6c06-0410-ace0-fbb531ad65f3