gdb: Disable -Werror for -Wmaybe-uninitialized

Newer GCCs are triggering false-positive -Wmaybe-uninitialized
warnings around code that uses gdb::optional:
  https://sourceware.org/ml/gdb-patches/2017-05/msg00118.html

Using std::optional wouldn't help, it triggers the same warnings:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80635

Initializing the variables to quiet the warning would defeat the
purpose of gdb::optional.  Making the optional ctor memset its storage
would be a pessimization.  Wrapping gdb::optional's internals with
"#pragma GCC diagnostic push/ignored/pop" doesn't work, we'd have to
wrap uses of gdb::optional instead, which I think would get unwieldy
and ugly as we start using gdb::optional more and more.

The -Wmaybe-uninitialized warning is documented as producing false
positives (unlike -Wuninialized), so until we find a better
workaround, disable -Werror for this warning.  You'll still see the
warning when building gdb, but it won't cause a build failure.

Tested by building with gcc 4.8.5, 5.3.1, and gcc trunk (20170428).

gdb/ChangeLog:
2017-05-05  Pedro Alves  <palves@redhat.com>

	* warning.m4 (build_warnings): Add -Wno-error=maybe-uninitialized.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2017-05-05  Pedro Alves  <palves@redhat.com>

	* configure: Regenerate.

1 files changed, 1 insertions, 1 deletions

diff --git a/gdb/configure b/gdb/configure
index 25895b154db..1554667dcb6 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -15108,7 +15108,7 @@ build_warnings="-Wall -Wpointer-arith \
 -Wno-unused -Wunused-value -Wunused-function \
 -Wno-switch -Wno-char-subscripts \
 -Wempty-body -Wunused-but-set-parameter -Wunused-but-set-variable \
--Wno-sign-compare -Wno-narrowing"
+-Wno-sign-compare -Wno-narrowing -Wno-error=maybe-uninitialized"
 
 # Enable -Wno-format by default when using gcc on mingw since many
 # GCC versions complain about %I64.