glr2.cc: disable GCC 4.6 warning

231. conflicts.at:1096: testing Syntax error in consistent error state: glr2.cc ...
    tests/conflicts.at:1096: $CXX $CXXFLAGS $CPPFLAGS  $LDFLAGS -o input input.cc $LIBS
    input.cc: In member function 'YYRESULTTAG glr_stack::yyresolveValue(glr_state*)':
    input.cc:2674:36: error: 'yysval' may be used uninitialized in this function [-Werror=uninitialized]

Do not initialize the variable: this way ASAN can really make sure we
do set it to a proper value.
If we initialize it, ASAN would report nothing.

* data/skeletons/c.m4 (YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN): Disable
GCC 4.6's -Wuninitialized.
* data/skeletons/glr2.cc: Disable the warning locally.

README-hacking.md

@@ -560,6 +560,26 @@ re-run the tests, run:
     make check
 
 
+## Docker
+
+Running old compilers is not very easy.  Docker can be used for some of
+them.  Have a look at .travis.yml for setups.  Move the tarball in /tmp and
+run, for instance:
+
+```
+docker run -v /tmp:/tmp -it ubuntu:xenial
+```
+
+### GCC 4.6
+On Ubuntu Xenial.
+
+```
+apt-get update
+apt-get install software-properties-common
+apt-add-repository -y "ppa:ubuntu-toolchain-r/test"
+apt-get update
+apt-get install -y gcc-4.6 g++-4.6 m4 make
+```
 
 Release Procedure
 =================