build: quiet annoying warnings without adding new ones

Disabling warnings can be tricky, because doing so can cause a different
compiler to create new warnings about unsupported disable flags. Also, some
warnings don't surface until they're paired with another warning (gcc). For
example, adding "-Wno-foo" won't cause any trouble, but if there's a legitimate
warning emitted, the "unknown option -Wno-foo" will show up as well.

Work around this in 2 ways:

1. When checking to see if -Wno-foo is supported, check for "-Wfoo" instead.
2. Enable -Werror while checking 1.

If "-Werror -Wfoo" compiles, "-Wno-foo" is almost guaranteed to be supported.

-Werror itself is also checked. If that fails to compile by itself, it likely
means that the user added a flag that adds a warning. In that case, -Werror
won't be used while checking, and the build may be extra noisy. The user would
need to fix the bad input flag.

Also, silence 2 more additional warnings that can show up post-c++11.

configure.ac

@@ -185,6 +185,9 @@ AC_ARG_ENABLE([debug],
     [enable_debug=$enableval],
     [enable_debug=no])
 
+AC_LANG_PUSH([C++])
+AX_CHECK_COMPILE_FLAG([-Werror],[CXXFLAG_WERROR="-Werror"],[CXXFLAG_WERROR=""])
+
 if test "x$enable_debug" = xyes; then
     CPPFLAGS="$CPPFLAGS -DDEBUG -DDEBUG_LOCKORDER"
     if test "x$GCC" = xyes; then
@@ -196,11 +199,19 @@ if test "x$enable_debug" = xyes; then
     fi
 fi
 
-## TODO: Remove these hard-coded paths and flags. They are here for the sake of
-##       compatibility with the legacy buildsystem.
-##
 if test "x$CXXFLAGS_overridden" = "xno"; then
-  CXXFLAGS="$CXXFLAGS -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter -Wno-self-assign"
+  AX_CHECK_COMPILE_FLAG([-Wall],[CXXFLAGS="$CXXFLAGS -Wall"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wextra],[CXXFLAGS="$CXXFLAGS -Wextra"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wformat],[CXXFLAGS="$CXXFLAGS -Wformat"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wformat-security],[CXXFLAGS="$CXXFLAGS -Wformat-security"],,[[$CXXFLAG_WERROR]])
+
+  ## Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
+  ## unknown options if any other warning is produced. Test the -Wfoo case, and
+  ## set the -Wno-foo case if it works.
+  AX_CHECK_COMPILE_FLAG([-Wunused-parameter],[CXXFLAGS="$CXXFLAGS -Wno-unused-parameter"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wself-assign],[CXXFLAGS="$CXXFLAGS -Wno-self-assign"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wunused-local-typedef],[CXXFLAGS="$CXXFLAGS -Wno-unused-local-typedef"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wdeprecated-register],[CXXFLAGS="$CXXFLAGS -Wno-deprecated-register"],,[[$CXXFLAG_WERROR]])
 fi
 CPPFLAGS="$CPPFLAGS -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS"
 
@@ -222,8 +233,6 @@ AC_ARG_WITH([daemon],
   [build_bitcoind=$withval],
   [build_bitcoind=yes])
 
-AC_LANG_PUSH([C++])
-
 use_pkgconfig=yes
 case $host in
   *mingw*)