CMake: fix test for warning suppressions

GCC doesn't warn for unknown `-Wno-` options, except if there are other
warnings or errors [0]. This was problematic with `CURL_WERROR` as that
warning-as-error cannot be suppressed. Notably, this always happened
with `-Wno-pedantic-ms-format` when not targeting Windows. So test for
the positive form of the warning instead, which should always result in
a diagnostic if unknown.

[0] https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

Closes

CMakeLists.txt

@@ -104,7 +104,7 @@ option(ENABLE_CURLDEBUG "Set to ON to build with TrackMemory feature enabled" OF
 
 if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_CLANG)
   if(PICKY_COMPILER)
-    foreach(_CCOPT -pedantic -Wall -W -Wpointer-arith -Wwrite-strings -Wunused -Wshadow -Winline -Wnested-externs -Wmissing-declarations -Wmissing-prototypes -Wno-long-long -Wfloat-equal -Wno-multichar -Wsign-compare -Wundef -Wno-format-nonliteral -Wendif-labels -Wstrict-prototypes -Wdeclaration-after-statement -Wstrict-aliasing=3 -Wcast-align -Wtype-limits -Wold-style-declaration -Wmissing-parameter-type -Wempty-body -Wclobbered -Wignored-qualifiers -Wconversion -Wno-sign-conversion -Wvla -Wdouble-promotion -Wno-system-headers -Wno-pedantic-ms-format)
+    foreach(_CCOPT -pedantic -Wall -W -Wpointer-arith -Wwrite-strings -Wunused -Wshadow -Winline -Wnested-externs -Wmissing-declarations -Wmissing-prototypes -Wfloat-equal -Wsign-compare -Wundef -Wendif-labels -Wstrict-prototypes -Wdeclaration-after-statement -Wstrict-aliasing=3 -Wcast-align -Wtype-limits -Wold-style-declaration -Wmissing-parameter-type -Wempty-body -Wclobbered -Wignored-qualifiers -Wconversion -Wvla -Wdouble-promotion)
       # surprisingly, CHECK_C_COMPILER_FLAG needs a new variable to store each new
       # test result in.
       string(MAKE_C_IDENTIFIER "OPT${_CCOPT}" _optvarname)
@@ -113,6 +113,15 @@ if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_CLANG)
         set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${_CCOPT}")
       endif()
     endforeach()
+    foreach(_CCOPT long-long multichar format-nonliteral sign-conversion system-headers pedantic-ms-format)
+      # GCC only warns about unknown -Wno- options if there are also other diagnostic messages,
+      # so test for the positive form instead
+      string(MAKE_C_IDENTIFIER "OPT${_CCOPT}" _optvarname)
+      check_c_compiler_flag("-W${_CCOPT}" ${_optvarname})
+      if(${_optvarname})
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-${_CCOPT}")
+      endif()
+    endforeach()
   endif()
 endif()