Don't hard-code the path of bash and use short options to run tar commands

This fixes the following problems when building on FreeBSD.

[1/2] Generating libstdc++.tag with a custom command.
FAILED: libstdc++.tag 
../../source/mm-common/util/meson_aux/libstdcxx-tag.sh curl /home/lantw44/gnome/source/mm-common/doctags libstdc++.tag
/bin/sh: ../../source/mm-common/util/meson_aux/libstdcxx-tag.sh: not found
[2/2] Generating skeletonmm.tar.xz with a custom command.
FAILED: skeletonmm.tar.xz 
../../source/mm-common/util/meson_aux/skeletonmm-tarball.sh /home/lantw44/gnome/source/mm-common skeletonmm.tar.xz skeletonmm/autogen.sh skeletonmm/.gitignore skeletonmm/AUTHORS skeletonmm/COPYING skeletonmm/ChangeLog skeletonmm/Makefile.am skeletonmm/README skeletonmm/configure.ac skeletonmm/skeletonmm.doap skeletonmm/build/.gitignore skeletonmm/codegen/generate_defs_and_docs.sh skeletonmm/codegen/Makefile.am skeletonmm/codegen/extradefs/generate_extra_defs_skeleton.cc skeletonmm/codegen/m4/convert.m4 skeletonmm/codegen/m4/convert_skeleton.m4 skeletonmm/codegen/m4/filelist.am skeletonmm/doc/Makefile.am skeletonmm/doc/reference/.gitignore skeletonmm/doc/reference/Doxyfile.in skeletonmm/examples/.gitignore skeletonmm/examples/Makefile.am skeletonmm/examples/example/example.cc skeletonmm/skeleton/.gitignore skeletonmm/skeleton/skeletonmm-uninstalled.pc.in skeletonmm/skeleton/skeletonmm.h skeletonmm/skeleton/skeletonmm.pc.in skeletonmm/skeleton/skeletonmmconfig.h.in skeletonmm/skeleton/skeletonmm/Makefile.am skeletonmm/skeleton/skeletonmm/filelist.am skeletonmm/skeleton/skeletonmm/wrap_init.h skeletonmm/skeleton/skeletonmm/private/.gitignore skeletonmm/skeleton/src/Makefile.am skeletonmm/skeleton/src/filelist.am skeletonmm/skeleton/src/skeleton.ccg skeletonmm/skeleton/src/skeleton.defs skeletonmm/skeleton/src/skeleton.hg skeletonmm/skeleton/src/skeleton_docs.xml skeletonmm/skeleton/src/skeleton_enum.defs skeletonmm/skeleton/src/skeleton_method.defs skeletonmm/skeleton/src/skeleton_signal.defs skeletonmm/skeleton/src/skeleton_vfunc.defs
/bin/sh: ../../source/mm-common/util/meson_aux/skeletonmm-tarball.sh: not found

[1/2] Generating skeletonmm.tar.xz with a custom command.
tar: Option --old-archive is not supported
Usage:
  List:    tar -tf <archive-filename>
  Extract: tar -xf <archive-filename>
  Create:  tar -cf <archive-filename> [filenames...]
  Help:    tar --help

I also tried the command tar -chof - on OpenBSD and NetBSD and it seemed to work fine.