configure should check for warning flags supported to disable them (#338205).

Configure would check whether gcc supported -Wno... flags. But gcc always
does. It is happy to just not warn about anything. So flip all configure
checks to test for the warning and only when gcc accepts the warning flag
use -Wno-...

Introduces two helper functions to make it easier to add new flag checks.
AC_GCC_WARNING_COND and AC_GCC_WARNING_SUBST_NO.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14319 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/NEWS b/NEWS
index 27312cc..d30aabc 100644
--- a/NEWS
+++ b/NEWS
@@ -210,6 +210,7 @@
 338024  inlined functions are not shown if DW_AT_ranges is used
 338115  DRD: computed conflict set differs from actual after fork
 338160  implement display of thread local storage in gdbsrv
+338205  configure.ac and check for -Wno-tautological-compare
 n-i-bz  Fix KVM_CREATE_IRQCHIP ioctl handling
 n-i-bz  s390x: Fix memory corruption for multithreaded applications
 n-i-bz  vex arm->IR: allow PC as basereg in some LDRD cases