libobs: Add OBS_UNIX_STRUCTURE config variable

This obsconfig.h variable will be true or false depending on whether the
project is configured with the cmake UNIX_STRUCTURE variable or not.

cmake/Modules/ObsHelpers.cmake

@@ -77,6 +77,7 @@ if(NOT UNIX_STRUCTURE)
 	endif()
 	set(OBS_CMAKE_DESTINATION "cmake")
 	set(OBS_INCLUDE_DESTINATION "include")
+	set(OBS_UNIX_STRUCTURE "0")
 else()
 	if(NOT OBS_MULTIARCH_SUFFIX AND DEFINED ENV{OBS_MULTIARCH_SUFFIX})
 		set(OBS_MULTIARCH_SUFFIX "$ENV{OBS_MULTIARCH_SUFFIX}")
@@ -98,6 +99,7 @@ else()
 	set(OBS_DATA_PATH "${OBS_DATA_DESTINATION}")
 	set(OBS_INSTALL_PREFIX "${CMAKE_INSTALL_PREFIX}/")
 	set(OBS_RELATIVE_PREFIX "../")
+	set(OBS_UNIX_STRUCTURE "1")
 endif()
 
 function(obs_finish_bundle)