Generate proper DL_OPENGL value for linux

Since we rely on the dynamic linker to find the library for us via
dlopen(), we need to have DL_OPENGL be .so.N, not the full library
filename, as ldconfig doesn't cache the full filename

Use of TARGET_SONAME_FILE requires the library to be marked as SHARED,
not MODULE

This closes pull request #370

cmake/Modules/ObsHelpers.cmake

@@ -521,10 +521,17 @@ function(define_graphic_modules target)
 	foreach(dl_lib opengl d3d9 d3d11)
 		string(TOUPPER ${dl_lib} dl_lib_upper)
 		if(TARGET libobs-${dl_lib})
-			target_compile_definitions(${target}
-				PRIVATE
-				DL_${dl_lib_upper}="$<TARGET_FILE_NAME:libobs-${dl_lib}>"
-				)
+			if(UNIX AND UNIX_STRUCTURE)
+				target_compile_definitions(${target}
+					PRIVATE
+					DL_${dl_lib_upper}="$<TARGET_SONAME_FILE_NAME:libobs-${dl_lib}>"
+					)
+			else()
+				target_compile_definitions(${target}
+					PRIVATE
+					DL_${dl_lib_upper}="$<TARGET_FILE_NAME:libobs-${dl_lib}>"
+					)
+			endif()
 		else()
 			target_compile_definitions(${target}
 				PRIVATE