Remove _THREAD_SAFE and _REENTRANT defines for non-threaded builds.

src/build/Common.gmake

@@ -299,7 +299,6 @@ ifeq ($(os),linux)    # Linux
  endif
 endif
 MT_CFLAGS    += -D_THREAD_SAFE -D_REENTRANT
-CFLAGS    += -D_THREAD_SAFE -D_REENTRANT -D__EXTENSIONS__
 ifeq ($(os),freebsd)    # FreeBSD
  MT_CFLAGS    += -DUSE_XP_SEMAPHORES
  MT_LDFLAGS    +=    -pthread
@@ -329,7 +328,7 @@ else
       MT_CFLAGS    +=    -D_POSIX_PTHREAD_SEMANTICS
       MT_CFLAGS    += -DUSE_XP_SEMAPHORES
       # This makes ctime_r() be the correct one.
-      CFLAGS	   += -D_POSIX_PTHREAD_SEMANTICS
+      #CFLAGS	   += -D_POSIX_PTHREAD_SEMANTICS
      else            # Linux / Other UNIX
       XP_SEM :=    1
       MT_CFLAGS    += -DUSE_XP_SEMAPHORES