Fix make install in pg_compile caused by incorrect SHELL

SHELL is an environment variable used in a couple of places mainly in
src/Makefile.global in PostgreSQL code to perform a couple of actions
like for example installing binaries. It happened that this does not
work correctly with msysgit and needed an enforcement to "bash". The
enforcement is made in a file not inside the PostgreSQL code itself
but in something generated, so that's something this code can obviously
live with.