configure: enable -Werror by default when available

Turn this on when building under Travis to increase coverage.

configure.ac

@@ -259,6 +259,13 @@ if test "$mingw_cv_win32_host" = yes; then
 AC_DEFINE([BGDWIN32], [], [Define is you are building for Win32 API])
 fi
 
+dnl Enable -Werror if possible.  Do it after all other tests.
+AC_ARG_ENABLE([werror], [AS_HELP_STRING([--enable-werror], [treat compile warnings as errors])])
+if test "$enable_werror" = "yes" || test "$GCC" = "yes" -a "$enable_werror" != "no"; then
+  CFLAGS="-Werror $CFLAGS"
+  CXXFLAGS="-Werror $CXXFLAGS"
+fi
+
 dnl report configuration
 AC_MSG_RESULT([
 ** Configuration summary for $PACKAGE $VERSION:

travis/main.sh

@@ -12,6 +12,7 @@ build_autotools() {
 	v --fold="configure" ./configure \
 		--prefix=/usr/local \
 		--libdir=/usr/local/lib \
+		--enable-werror \
 		--with-fontconfig \
 		--with-freetype \
 		--with-jpeg \