linux-build: -std=gnu extensions + fix remaining snprintf decls

- CMakeLists: CMAKE_C_EXTENSIONS/CXX_EXTENSIONS ON. Strict -std=c11/c++20
  define __STRICT_ANSI__, which disables glibc _DEFAULT_SOURCE and hides
  clock_gettime(CLOCK_REALTIME) etc. that port/src/libultra.c needs.
  -std=gnu11/gnu++20 is GCC's default and what MinGW already used.
- port/src/{pccg,pcmodels,romdata}.c: local 'extern int snprintf(...,
  unsigned long long maxsize, ...)' decls conflict with glibc's size_t one
  (same bug already fixed in pc_protos.h in M-39); spell it size_t.
- libultra.c: drop the _GNU_SOURCE define (pulls cpu_set_t refs into
  <pthread.h> without <sched.h>); the D60 POSIX stack-bounds diagnostic
  now just prints n/a rather than needing pthread_getattr_np.

Windows: full rebuild 240/241, -level_09 crash-free past frame 1740.

CMakeLists.txt

@@ -52,9 +52,13 @@ include(TargetArch)
 include(FindPkgConfig)
 
 set(CMAKE_C_STANDARD 11)
-set(CMAKE_C_EXTENSIONS OFF)
+# GNU extensions ON: strict -std=c11/-std=c++20 define __STRICT_ANSI__, which
+# disables glibc's _DEFAULT_SOURCE and hides POSIX symbols the port needs
+# (clock_gettime/CLOCK_REALTIME in port/src/libultra.c, mmap flags, etc.).
+# -std=gnu11/gnu++20 is GCC's own default and what MinGW was already using.
+set(CMAKE_C_EXTENSIONS ON)
 set(CMAKE_CXX_STANDARD 20)
-set(CMAKE_CXX_EXTENSIONS OFF)
+set(CMAKE_CXX_EXTENSIONS ON)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
 # Default build type