fix(warnings): silence two -Wall/-Wextra warnings on the MinGW build

A clean 'make glm' on MinGW emitted exactly two warnings, both real:

1. compat.h:240 - ignoring pragma comment [-Wunknown-pragmas]
   #pragma comment(lib, "psapi.lib") is an MSVC directive; MinGW/GCC
   warns about it. Guarded with ifdef _MSC_VER - MinGW links psapi via
   -lpsapi (already in the Makefile), MSVC keeps the pragma.

2. glm.c:1210 - g_numa_nodes defined but not used [-Wunused-variable]
   g_numa_nodes is only read/written inside ifdef __linux__ blocks, so on
   every non-Linux build it is a static that is never used. Moved the
   definition under the same __linux__ guard; nothing references it off-Linux.

Verified: rm -f *.o glm.exe && make glm -> 0 warnings, 0 errors.

c/compat.h

@@ -237,7 +237,9 @@ static inline int compat_rename(const char *old, const char *new){
 /* --- rss_gb: getrusage -> GetProcessMemoryInfo ---
  * ru_maxrss in KB (come Linux): rss_gb() divide per 1e6 → GB corretti. */
 #include <psapi.h>
-#pragma comment(lib, "psapi.lib")
+#ifdef _MSC_VER
+#pragma comment(lib, "psapi.lib")   /* MSVC: link psapi; MinGW/GCC uses -lpsapi */
+#endif
 struct rusage { long ru_maxrss; };
 #define RUSAGE_SELF 0
 static inline int getrusage(int who, struct rusage *r){