BUILD: Test for -Wfoobar instead of -Wno-foobar

gcc is more tolerant at allowing stray, unsupported -Wno-foobar.
So if we want to test whether a compiler support -Wno-foobar, we need
to test for -Wfoobar instead.

configure.ac

@@ -45,7 +45,7 @@ if test "x$with_warnings" = "xno"; then
 	WARN_C_CXX=""
 	WARN_CXX=""
 
-	AX_CHECK_COMPILER_FLAGS_VAR([C], [WARN_C_CXX], [-Wno-deprecated-declarations])
+	AX_CHECK_COMPILER_FLAGS_VAR([C], [WARN_C_CXX], [-Wdeprecated-declarations], [-Wno-deprecated-declarations])
 else
 	WARN_C_CXX="-Wall"
 	AX_CHECK_COMPILER_FLAGS_VAR([C], [WARN_C_CXX], [-Wignored-qualifiers])
@@ -56,13 +56,13 @@ else
 	AX_CHECK_COMPILER_FLAGS_VAR([C], [WARN_C_CXX], [-Wuninitialized])
 	AX_CHECK_COMPILER_FLAGS_VAR([C], [WARN_C_CXX], [-Wunused-parameter])
 	AX_CHECK_COMPILER_FLAGS_VAR([C], [WARN_C_CXX], [-Wunused-but-set-parameter])
-	AX_CHECK_COMPILER_FLAGS_VAR([C], [WARN_C_CXX], [-Wno-deprecated-declarations])
-	AX_CHECK_COMPILER_FLAGS_VAR([C], [WARN_C_CXX], [-Wno-long-long])
+	AX_CHECK_COMPILER_FLAGS_VAR([C], [WARN_C_CXX], [-Wdeprecated-declarations], [-Wno-deprecated-declarations])
+	AX_CHECK_COMPILER_FLAGS_VAR([C], [WARN_C_CXX], [-Wlong-long], [-Wno-long-long])
 
 	WARN_CXX=""
 	AX_CHECK_COMPILER_FLAGS_VAR([C++], [WARN_CXX], [-Wnon-virtual-dtor])
-	AX_CHECK_COMPILER_FLAGS_VAR([C++], [WARN_CXX], [-Wno-overloaded-virtual])
-	AX_CHECK_COMPILER_FLAGS_VAR([C++], [WARN_CXX], [-Wno-c++11-long-long])
+	AX_CHECK_COMPILER_FLAGS_VAR([C++], [WARN_CXX], [-Woverloaded-virtual], [-Wno-overloaded-virtual])
+	AX_CHECK_COMPILER_FLAGS_VAR([C++], [WARN_CXX], [-Wc++11-long-long], [-Wno-c++11-long-long])
 fi
 
 AC_SUBST(WARN_C_CXX)

m4/ax_check_compiler_flags_var.m4

@@ -1,8 +1,12 @@
 #serial 1000
-dnl @synopsis AX_CHECK_COMPILER_FLAGS_VAR(language, variable, flag)
+dnl @synopsis AX_CHECK_COMPILER_FLAGS_VAR(language, variable, flag-to-test, flag-to-use)
 dnl
-dnl @summary Check whether the compiler for this language accepts this flag, and
-dnl          add it to the variable if it does
+dnl @summary Check whether the compiler for this language accepts flag-to-test,
+dnl          and add flag-to-add to the variable if it does. flag-to-test and
+dnl          flag-to-add can be different, because a compiler might allow
+dnl          a stray -Wno-foobar, while not actually supporting it. In this
+dnl          case, flag-to-test should be -Wfoobar. If flag-to-use is empty
+dnl          flag-to-test will be used.
 dnl
 dnl Depends on the AX_CHECK_COMPILER_FLAGS macro.
 dnl
@@ -14,7 +18,12 @@ dnl @license Creative Commons CC0 1.0 Universal Public Domain Dedication
 AC_DEFUN([AX_CHECK_COMPILER_FLAGS_VAR], [
 	AC_LANG_PUSH([$1])
 
-	AX_CHECK_COMPILER_FLAGS([$3], [-Werror], AS_VAR_APPEND([$2], [" $3"]))
+	FLAG_TO_USE="$4"
+	if test -z "$FLAG_TO_USE"; then
+		FLAG_TO_USE="$3"
+	fi
+
+	AX_CHECK_COMPILER_FLAGS([$3], [-Werror], AS_VAR_APPEND([$2], [" $FLAG_TO_USE"]))
 
 	AC_LANG_POP([$1])
 ])