libobs: Fix `LINUX_PORTABLE` preprocessor macro usage

To simplify logic in source files, `LINUX_PORTABLE` is only defined
if the associated build flag is enabled. Thus straight-forward `ifdef`
checks suffice.

libobs/obsconfig.h.in

@@ -15,7 +15,7 @@
 #define OBS_INSTALL_PREFIX "@OBS_INSTALL_PREFIX@"
 #define OBS_PLUGIN_DESTINATION "@OBS_PLUGIN_DESTINATION@"
 
-#define LINUX_PORTABLE "@LINUX_PORTABLE@"
+#cmakedefine LINUX_PORTABLE
 #cmakedefine GIO_FOUND
 #cmakedefine PULSEAUDIO_FOUND
 #cmakedefine XCB_XINPUT_FOUND