CMakeLists: do not enforce strict prototypes on Windows.

On Windows, or at least when cross-built with Mingw-w64, build fails
because strict prototype fails on an included file (thus nothing we can
do about in json-c code):

> from /home/jehan/dev/src/json-c/json_util.c:44:
> /home/jehan/.local/share/crossroad/roads/w64/json-c/include/minwindef.h:196:3: error: function declaration isn't a prototype [-Werror=strict-prototypes]
>   196 |   typedef INT_PTR (WINAPI *FARPROC) ();
>       |   ^~~~~~~
> /home/jehan/.local/share/crossroad/roads/w64/json-c/include/minwindef.h:197:3: error: function declaration isn't a prototype [-Werror=strict-prototypes]
>   197 |   typedef INT_PTR (WINAPI *NEARPROC) ();
>       |   ^~~~~~~
> /home/jehan/.local/share/crossroad/roads/w64/json-c/include/minwindef.h:198:3: error: function declaration isn't a prototype [-Werror=strict-prototypes]
>   198 |   typedef INT_PTR (WINAPI *PROC) ();
>       |   ^~~~~~~

Let's just disable the errors for Windows build.

CMakeLists.txt

@@ -240,7 +240,9 @@ if ("${CMAKE_C_COMPILER_ID}" STREQUAL "GNU")
     set(CMAKE_C_FLAGS           "${CMAKE_C_FLAGS} -Wextra")
     set(CMAKE_C_FLAGS           "${CMAKE_C_FLAGS} -Wwrite-strings")
     set(CMAKE_C_FLAGS           "${CMAKE_C_FLAGS} -Wno-unused-parameter")
-    set(CMAKE_C_FLAGS           "${CMAKE_C_FLAGS} -Wstrict-prototypes")
+    if (NOT WIN32)
+        set(CMAKE_C_FLAGS           "${CMAKE_C_FLAGS} -Wstrict-prototypes")
+    endif()
 
     add_definitions(-D_GNU_SOURCE)
 elseif ("${CMAKE_C_COMPILER_ID}" STREQUAL "MSVC")