build: speed up configure for releases

build: speed up configure for releases

Since most users won't be building with GNULIB_POSIXCHECK defined in
CFLAGS, and since we can make ./configure 10% (several seconds!) faster
by omitting the framework for a posix check, this patch makes it so
that the framework is omitted by default, while still giving
instructions for maintainers to re-enable it.

It's been a while since we've used GNULIB_POSIXCHECK; see this email:
https://lists.gnu.org/archive/html/coreutils/2012-03/msg00126.html
Some of those failures are because we are intentionally avoiding
specific gnulib modules (that is, we have chosen not to use things
like fprintf-posix), but until we work with gnulib to avoid particular
warnings, wiring up an automatic GNULIB_POSIXCHECK to happen during
'make my-distcheck' is not feasible.

* configure.ac (gl_ASSERT_NO_GNULIB_POSIXCHECK): Conditionally
define, according to whether $GNULIB_POSIXCHECK is in environment.