Move defines into CMakeLists.txt

These aren't dynamically detected.

CMakeLists.txt

@@ -63,3 +63,6 @@ add_library(xpdevplus STATIC ${SOURCE})
 target_link_libraries(xpdevplus ciolib comio encode hash uifc xpdev)
 target_include_directories(xpdevplus PUBLIC ${XPDevPlus_BINARY_DIR})
 target_compile_options(xpdevplus PUBLIC "SHELL:-include xpdevplus.h")
+if (CMAKE_SYSTEM_NAME STREQUAL "Linux")
+target_compile_options(xpdevplus PUBLIC POSIX_C_SOURCE 200809L _DEFAULT_SOURCE _BSD_SOURCE SPEED_MACROS_ONLY _GNU_SOURCE _FILE_OFFSET_BITS 64)
+endif()

xpdevplus.h.in

 #ifndef XPDEV_PLUS_H
 #define XPDEV_PLUS_H
 
-#ifdef __linux__
-#define POSIX_C_SOURCE 200809L
-#define _DEFAULT_SOURCE
-#define _BSD_SOURCE
-#define SPEED_MACROS_ONLY 
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE 
-#endif
-#define _FILE_OFFSET_BITS 64
-#endif
-
 #cmakedefine WITH_SDL
 #cmakedefine N_CURSES_LIB
 #cmakedefine HAS_INTTYPES_H