configure: add --enable-werror

Provide a sane means of adding the -Werror compiler flag.

Currently, some of our jenkins.sh add -Werror by passing 'CFLAGS="-Werror"',
but that actually *overwrites* all the other CFLAGS we might want to have set.

Maintain these exceptions from -Werror:
a) deprecation (allow upstream to mark deprecation without breaking builds);
b) "#warning" pragmas (allow to remind ourselves of errors without breaking
   builds)

As a last configure step before generating the output files, print the complete
CFLAGS and CPPFLAGS by means of AC_MSG_RESULT.

Change-Id: Ibcc8238584a585434b39a046cd2d7e18ddaf7f8c

configure.ac

@@ -29,6 +29,24 @@ then
 	CPPFLAGS="$CPPFLAGS -fsanitize=address -fsanitize=undefined"
 fi
 
+AC_ARG_ENABLE(werror,
+	[AS_HELP_STRING(
+		[--enable-werror],
+		[Turn all compiler warnings into errors, with exceptions:
+		 a) deprecation (allow upstream to mark deprecation without breaking builds);
+		 b) "#warning" pragmas (allow to remind ourselves of errors without breaking builds)
+		]
+	)],
+	[werror=$enableval], [werror="no"])
+if test x"$werror" = x"yes"
+then
+	WERROR_FLAGS="-Werror"
+	WERROR_FLAGS+=" -Wno-error=deprecated -Wno-error=deprecated-declarations"
+	WERROR_FLAGS+=" -Wno-error=cpp" # "#warning"
+	CFLAGS="$CFLAGS $WERROR_FLAGS"
+	CPPFLAGS="$CPPFLAGS $WERROR_FLAGS"
+fi
+
 # The following test is taken from WebKit's webkit.m4
 saved_CFLAGS="$CFLAGS"
 CFLAGS="$CFLAGS -fvisibility=hidden "
@@ -44,6 +62,9 @@ AC_ARG_ENABLE([debug], [AS_HELP_STRING([--enable-debug], [Buld the library with
 		[build_debug="$enableval"], [build_debug="no"])
 AM_CONDITIONAL(BUILD_DEBUG, test "x$build_debug" = "xyes")
 
+AC_MSG_RESULT([CFLAGS="$CFLAGS"])
+AC_MSG_RESULT([CPPFLAGS="$CPPFLAGS"])
+
 AC_OUTPUT(
         libasn1c.pc
         src/Makefile