It's spelt _POSIX_C_SOURCE

Since this doesn't work, we're clearly not relying on it, so remove
it.

Reported by: jrtc27 via IRC

src/build/Common.gmake

@@ -412,7 +412,7 @@ endif
 
 # PThread-specific flags
 ifeq ($(os),linux)    # Linux
- CFLAGS	+=	-DPOSIX_C_SOURCE=200809L -D_DEFAULT_SOURCE -D_BSD_SOURCE -DSPEED_MACROS_ONLY -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64
+ CFLAGS	+=	-D_DEFAULT_SOURCE -D_BSD_SOURCE -DSPEED_MACROS_ONLY -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64
  ifndef THREADS_ACTUALLY_WORK
   CFLAGS    += -D_THREAD_SUID_BROKEN
  endif

src/xpdev/CMakeLists.txt

@@ -154,7 +154,7 @@ elseif(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
 	target_compile_definitions(xpdev PUBLIC __unix__ __DARWIN__ USE_SNPRINTF)
 	target_link_libraries(xpdev m)
 elseif(CMAKE_SYSTEM_NAME STREQUAL "Linux")
-	target_compile_definitions(xpdev PUBLIC POSIX_C_SOURCE=200809L _DEFAULT_SOURCE _BSD_SOURCE SPEED_MACROS_ONLY _GNU_SOURCE _FILE_OFFSET_BITS=64)
+	target_compile_definitions(xpdev PUBLIC _DEFAULT_SOURCE _BSD_SOURCE SPEED_MACROS_ONLY _GNU_SOURCE _FILE_OFFSET_BITS=64)
 	target_link_libraries(xpdev m)
 else()
 	target_link_libraries(xpdev m)