Do not use PKG_CHECK_MODULES in configure

Do not use PKG_CHECK_MODULES in configure

Issue #200 (bdwgc).

Presence of libatomic_ops is now detected only by checking the
installed headers, but libatomic_ops is no longer needed for most of
the clients which have atomic intrinsics support by the compiler.

* README.md (Installation and Portability): Remove information about
"syntax error near unexpected token ATOMIC_OPS".
* autogen.sh: Update comment removing pkg-config from the requirements.
* configure.ac (PKG_CHECK_MODULES(ATOMIC_OPS)): Comment out; update the
comment.