New configure option: --enable-gcc-warnings-advisory

the new configure option --enable-gcc-warnings-advisory enables
all the gcc flags that --enable-gcc-warnings does with the
exception of -Werror.

configure.in

@@ -95,6 +95,8 @@ fi
 
 AC_ARG_ENABLE(gcc-warnings,
      AS_HELP_STRING(--enable-gcc-warnings, enable verbose warnings))
+AC_ARG_ENABLE(gcc-warnings-advisory,
+     AS_HELP_STRING(--enable-gcc-warnings-advisory, [enable verbose warnings, excluding -Werror]))
 
 AC_ARG_ENABLE(local-appdata,
    AS_HELP_STRING(--enable-local-appdata, default to host local application data paths on Windows))
@@ -762,11 +764,12 @@ if test "$GCC" = yes; then
 else
   CFLAGS="$CFLAGS -g -O"
   enable_gcc_warnings=no
+  enable_gcc_warnings_advisory=no
 fi
 
 # Add some more warnings which we use in development but not in the
 # released versions.  (Some relevant gcc versions can't handle these.)
-if test x$enable_gcc_warnings = xyes; then
+if test x$enable_gcc_warnings = xyes || test x$enable_gcc_warnings_advisory = xyes; then
 
   AC_COMPILE_IFELSE(AC_LANG_PROGRAM([], [
 #if !defined(__GNUC__) || (__GNUC__ < 4)
@@ -789,7 +792,10 @@ if test x$enable_gcc_warnings = xyes; then
                     have_shorten64_flag=no)
   CFLAGS="$save_CFLAGS"
 
-  CFLAGS="$CFLAGS -W -Wfloat-equal -Wundef -Wpointer-arith -Wstrict-prototypes -Wmissing-prototypes -Wwrite-strings -Wredundant-decls -Wchar-subscripts -Wcomment -Wformat=2 -Wwrite-strings -Wmissing-declarations -Wredundant-decls -Wnested-externs -Wbad-function-cast -Wswitch-enum -Werror"
+  CFLAGS="$CFLAGS -W -Wfloat-equal -Wundef -Wpointer-arith -Wstrict-prototypes -Wmissing-prototypes -Wwrite-strings -Wredundant-decls -Wchar-subscripts -Wcomment -Wformat=2 -Wwrite-strings -Wmissing-declarations -Wredundant-decls -Wnested-externs -Wbad-function-cast -Wswitch-enum"
+  if test x$enable_gcc_warnings = xyes; then
+    CFLAGS="$CFLAGS -Werror"
+  fi
 
   # Disabled, so we can use mallinfo(): -Waggregate-return