cmake: do all compile tests with -D_GNU_SOURCE=1

[madebr]

Fixes #7026

[sezero]

SDL2 (and release-2.26.x) branches need a similar fix, although I'm not sure whether it's worth it.

[madebr]

Pushed as 460b7b2 and e312fbb

[sezero]

Assuming CMAKE_REQUIRED_FLAGS affects check_c_source_compiles,
is the following OK as a follow-up cleanup?

diff --git a/cmake/sdlchecks.cmake b/cmake/sdlchecks.cmake
index ee24d57..8700441 100644
--- a/cmake/sdlchecks.cmake
+++ b/cmake/sdlchecks.cmake
@@ -820,7 +820,6 @@ macro(CheckPTHREAD)
       list(APPEND SDL_PC_CFLAGS ${PTHREAD_CFLAGS})
 
       check_c_source_compiles("
-        #define _GNU_SOURCE 1
         #include <pthread.h>
         int main(int argc, char **argv) {
           pthread_mutexattr_t attr;
@@ -831,7 +830,6 @@ macro(CheckPTHREAD)
         set(SDL_THREAD_PTHREAD_RECURSIVE_MUTEX 1)
       else()
         check_c_source_compiles("
-            #define _GNU_SOURCE 1
             #include <pthread.h>
             int main(int argc, char **argv) {
               pthread_mutexattr_t attr;
@@ -862,7 +860,6 @@ macro(CheckPTHREAD)
       check_include_file(pthread_np.h HAVE_PTHREAD_NP_H)
       if (HAVE_PTHREAD_H)
         check_c_source_compiles("
-            #define _GNU_SOURCE 1
             #include <pthread.h>
             int main(int argc, char **argv) {
               pthread_setname_np(pthread_self(), \"\");

[madebr]

Yes, that should be fine!