Fix probable compile errors due to redefining macros

- The macro _POSIX_C_SOURCE may be defined by compilers,
  a value incompatible with that specified in the source.
  This may trigger compiler warnings / errors depending
  on the configuration. We undefine it and redefine
  it to the value that we want.

- The macro _GNU_SOURCE may be undefined in the future,
  depending on which direction ola development progresses.
  It is only undefined when the macro is present to avoid
  future errors.

Author: shenghaoyang

olad/Strerror_r.cpp

@@ -24,8 +24,16 @@
 
 // Required for string.h to declare the standards-compliant version of
 // strerror_r(), when compiling under glibc. Must come before the inclusion
-// of string.h
+// of string.h.
+// These are conditional to avoid errors when not compiling with glibc, or
+// when compiling with a compiler that does not define _POSIX_C_SOURCE.
+#ifdef _GNU_SOURCE
 #undef _GNU_SOURCE
+#endif
+
+#ifdef _POSIX_C_SOURCE
+#undef _POSIX_C_SOURCE
+#endif
 #define _POSIX_C_SOURCE 200112L
 
 #include <string.h>