Add -Werror into --enable-developer-flags

I no longer get any warnings from mongoc or libbson using these flags
so lets try this out!

Author: bjori

config.m4

@@ -109,6 +109,7 @@ if test "$MONGODB" != "no"; then
     PHP_CHECK_GCC_ARG(-Wempty-body,                     _MAINTAINER_CFLAGS="$_MAINTAINER_CFLAGS -Wempty-body")
     PHP_CHECK_GCC_ARG(-Wparentheses,                    _MAINTAINER_CFLAGS="$_MAINTAINER_CFLAGS -Wparentheses")
     PHP_CHECK_GCC_ARG(-Wdeclaration-after-statement,    _MAINTAINER_CFLAGS="$_MAINTAINER_CFLAGS -Wdeclaration-after-statement")
+    PHP_CHECK_GCC_ARG(-Werror,                          _MAINTAINER_CFLAGS="$_MAINTAINER_CFLAGS -Werror")
 
 
     MAINTAINER_CFLAGS="-Wextra $_MAINTAINER_CFLAGS  -Wno-unused-parameter -Wno-unused-but-set-variable -Wno-missing-field-initializers"