Add '-Wall -Werror' to the compile options in the autotools makefile,

having just noticed that Makefile.gtk had it and this one doesn't. (Of
course, this being autoconf, we can easily enough make it conditional
on the compiler actually being gcc.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@9583 cda61777-01e9-0310-a592-d414129be87e