[PATCHv2] build: speed up configure for releases

[Eric Blake]

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 happend during
'make my-distcheck' is not feasible.

* configure.ac (gl_ASSERT_NO_GNULIB_POSIXCHECK): Conditionally
define, according to whether $GNULIB_POSIXCHECK is in environment.
---
 configure.ac |   14 ++++++++++++++
 1 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/configure.ac b/configure.ac
index 07597e6..5a4860e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -35,6 +35,20 @@ AC_CONFIG_HEADERS([lib/config.h:lib/config.hin])
 AM_INIT_AUTOMAKE([1.11.1 no-dist-gzip dist-xz color-tests parallel-tests])
 AM_SILENT_RULES([yes]) # make --enable-silent-rules the default.

+dnl POSIXCHECK is worthwhile for maintainers, but adds several seconds
+dnl (more than 10% execution time) to ./configure, with no benefit for
+dnl most users.  Using it to look for bugs requires:
+dnl   GNULIB_POSIXCHECK=1 autoreconf -f
+dnl   ./configure
+dnl   make
+dnl   make -C src clean
+dnl   make CFLAGS=-DGNULIB_POSIXCHECK=1
+dnl FIXME: Once we figure out how to avoid false positives, we should
+dnl have 'make my-distcheck' in dist-check.mk exercise this.
+m4_syscmd([test "${GNULIB_POSIXCHECK+set}" = set])
+m4_if(m4_sysval, [0], [], [dnl
+gl_ASSERT_NO_GNULIB_POSIXCHECK])
+
 AC_PROG_CC_STDC
 AM_PROG_CC_C_O
 AC_PROG_CPP
-- 
1.7.7.6