build: remove -Wformat-truncation=2 to avoid avoid false-positive

* configure.ac (GNULIB_WARN_CFLAGS): Add -Wformat-truncation=2,
to disable it when building gnulib with --enable-gcc-warnings.
This avoids what looks like a false positive from GCC 9:
strerror_r.c: In function 'rpl_strerror_r':
strerror_r.c:453:35: error: 'Unknown error ' directive output \
 truncated writing 14 bytes into a region of size 2 \
 [-Werror=format-truncation=]
         snprintf (buf, buflen, "Unknown error %d", errnum);
                                 ~~^~~~~~~~~~~~
In file included from /usr/include/stdio.h:862,
                 from ./stdio.h:43,
                 from strerror_r.c:29:
/usr/include/bits/stdio2.h:64:10: note: '__builtin___snprintf_chk' \
  output between 16 and 26 bytes into a destination of size 2
   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,

1 files changed, 1 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index 1f05a7e..361d877 100644
--- a/configure.ac
+++ b/configure.ac
@@ -177,6 +177,7 @@ if test "$gl_gcc_warnings" = yes; then
   # Remove the following and save the result in GNULIB_WARN_CFLAGS.
   nw=
   nw="$nw -Wstrict-overflow"
+  nw="$nw -Wformat-truncation=2"
   nw="$nw -Wuninitialized"
   nw="$nw -Wunused-macros"
   nw="$nw -Wmissing-prototypes"