Only enable linux stacktraces when using glibc (#69)

This is a follow-up to previous attempt #90
Stacktraces use glibc-specific extension execinfo.h, so currently loguru
compilation will fail with other C libraries like musl with an error
like:

loguru/loguru.cpp:103:18: fatal error: execinfo.h: No such file or directory
  103 |         #include <execinfo.h>  // for backtrace

This change adds a check on __GLIBC__ before enabling backtraces, and
simplifies the logic around it a bit: first checking Windows, then
glibc, then other systems

loguru.cpp

@@ -75,23 +75,24 @@
 
 // TODO: use defined(_POSIX_VERSION) for some of these things?
 
+
 #if defined(_WIN32) || defined(__CYGWIN__)
  #define LOGURU_PTHREADS 0
  #define LOGURU_WINTHREADS 1
  #ifndef LOGURU_STACKTRACES
  #define LOGURU_STACKTRACES 0
  #endif
-#elif defined(__rtems__) || defined(__ANDROID__) || defined(__FreeBSD__)
- #define LOGURU_PTHREADS 1
- #define LOGURU_WINTHREADS 0
- #ifndef LOGURU_STACKTRACES
- #define LOGURU_STACKTRACES 0
- #endif
 #else
  #define LOGURU_PTHREADS 1
  #define LOGURU_WINTHREADS 0
- #ifndef LOGURU_STACKTRACES
- #define LOGURU_STACKTRACES 1
+ #ifdef __GLIBC__
+ #ifndef LOGURU_STACKTRACES
+ #define LOGURU_STACKTRACES 1
+ #endif
+ #else
+ #ifndef LOGURU_STACKTRACES
+ #define LOGURU_STACKTRACES 0
+ #endif
  #endif
 #endif