Work with beta autoconf 2.50.

Work with beta autoconf 2.50.
* configure.in (GLIB_MAJOR_VERSION): Use GLIB_DIVERT_BEFORE_HELP
instead of AC_DIVERT_PUSH(),AC_DIVERT_POP.
(AC_CHECK_HEADERS): Remove redundant AC_DEFINE(HAVE_...).
(REALLOC_0_WORKS): Move AC_DEFINE outside AC_CACHE_VAL.
(dlopen): Quote nested AC_CHECK_... calls.

* acglib.m4 (GLIB_TR_SH, GLIB_TR_CPP): Utility macros copied from
beta autoconf 2.50.
(GLIB_DIVERT_BEFORE_HELP): New macro that works both with autoconf
2.13 and beta autoconf 2.50.
(GLIB_SIZEOF, GLIB_BYTECONTENTS): Use GLIB_TR_*.