cmake: Fix checking compiler flags like -Wno-some-warning

[hebasto]

Some compilers (GCC) produce no diagnostic for -Wno-unknown-warning unless other diagnostics are being produced:

$ git diff
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 240557f..f976824 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -220,6 +220,7 @@ if(MSVC)
   add_compile_definitions(_CRT_SECURE_NO_WARNINGS)
 else()
   # Keep the following commands ordered lexicographically.
+  try_append_c_flags(-Wno-unknown-warning)
   try_append_c_flags(-pedantic)
   try_append_c_flags(-Wall) # GCC >= 2.95 and probably many other compilers.
   try_append_c_flags(-Wcast-align) # GCC >= 2.95.
$ cmake -B build -DCMAKE_C_COMPILER=gcc 2>&1 | grep -i unknown
-- Performing Test C_SUPPORTS__WNO_UNKNOWN_WARNING
-- Performing Test C_SUPPORTS__WNO_UNKNOWN_WARNING - Success
Compile options ....................... -Wno-unknown-warning -pedantic -Wall -Wcast-align -Wcast-align=strict -Wextra -Wnested-externs -Wno-long-long -Wno-overlength-strings -Wno-unused-function -Wshadow -Wstrict-prototypes -Wundef

This PR fixes compiler flag check in CMake-based build system.

Let me know, if the same fix is desirable in Autotools-based build system.