Support a shell other than /bin/sh as a workaround for MacOS X.

Under MacOS X the LD_LIBRARY_PATH variable is unset by the build system
running executables under /bin (namely /bin/sh).  This is part of MacOS X
System Integrity Protection.

Because many (most?) users of ninja/meson seem to need hacks involving
LD_LIBRARY_PATH, use a non-SIP-triggering shell under MacOS X.

Fixes MacOS X builds of glib2, gobject-introspection and others.

Bump PKGREVISION for the new behaviour under MacOS X.

(dsainty)