configure: Properly check for availability of -Wno-foo warning flags

For some weird reason gcc does not check if the -Wno disabling variants
of warning flags match existing warning flags. Instead it swallows them
silently. That is, unless other warning or error messages are generated,
because then - for some even more bizarre reason - a complaint about the
unknown disable warning flag is issued along with the error or warning
message.

Thus to check for the availability of a warning disabling option, one
needs to check for the enabling variant instead and then add the
disabling variant to CFLAGS.

configure

@@ -3903,13 +3903,9 @@ enabled debug && add_cflags -g"$debuglevel" && add_asflags -g"$debuglevel"
 # add some useful compiler flags if supported
 check_cflags -Wdeclaration-after-statement
 check_cflags -Wall
-check_cflags -Wno-parentheses
-check_cflags -Wno-switch
-check_cflags -Wno-format-zero-length
 check_cflags -Wdisabled-optimization
 check_cflags -Wpointer-arith
 check_cflags -Wredundant-decls
-check_cflags -Wno-pointer-sign
 check_cflags -Wcast-qual
 check_cflags -Wwrite-strings
 check_cflags -Wtype-limits
@@ -3918,6 +3914,17 @@ check_cflags -Wmissing-prototypes
 check_cflags -Wstrict-prototypes
 enabled extra_warnings && check_cflags -Winline
 
+check_disable_warning(){
+    warning_flag=-W${1#-Wno-}
+    test_cflags $warning_flag && add_cflags $1
+}
+
+check_disable_warning -Wno-parentheses
+check_disable_warning -Wno-switch
+check_disable_warning -Wno-format-zero-length
+check_disable_warning -Wno-pointer-sign
+enabled extra_warnings || check_disable_warning -Wno-maybe-uninitialized
+
 # add some linker flags
 check_ldflags -Wl,--warn-common
 check_ldflags -Wl,-rpath-link=libswscale:libavfilter:libavdevice:libavformat:libavcodec:libavutil:libavresample
@@ -4008,7 +4015,6 @@ elif enabled gcc; then
     check_cflags -Werror=return-type
     check_cflags -Werror=declaration-after-statement
     check_cflags -Werror=vla
-    enabled extra_warnings || check_cflags -Wno-maybe-uninitialized
 elif enabled llvm_gcc; then
     check_cflags -mllvm -stack-alignment=16
 elif enabled clang; then