Make it workable on OpenBSD

- Additional LMMS_BUILD flag.
- Disallow on plugins -Wl,-no-undefined which triggers undefined references.
- Make sure X11 headers are found.

Lib ossaudio is needed only for OpenBSD

redundant expression removal

simplify condition for detection OS 'kind'

seems the last commit brought an issue on OSx travis test ....

cmake/modules/DetectMachine.cmake

@@ -2,6 +2,8 @@ IF(WIN32)
 	SET(LMMS_BUILD_WIN32 1)
 ELSEIF(APPLE)
 	SET(LMMS_BUILD_APPLE 1)
+ELSEIF(${CMAKE_SYSTEM_NAME} MATCHES "OpenBSD")
+    SET(LMMS_BUILD_OPENBSD 1)
 ELSEIF(HAIKU)
 	SET(LMMS_BUILD_HAIKU 1)
 ELSE()