fix backtrace detection in CMake

works on arch (glibc) and void linux (tested with musl libc) now

CMakeLists.txt

@@ -61,9 +61,9 @@ find_package(PAM)
 
 find_package(Backtrace)
 if(Backtrace_FOUND)
-  include_directories(${Backtrace_INCLUDE_DIRS})
-  target_link_libraries(${Backtrace_LIBRARIES})
+  include_directories("${Backtrace_INCLUDE_DIRS}")
   add_definitions(-DSWAY_Backtrace_FOUND=1)
+  set(LINK_LIBRARIES, "${LINK_LIBRARIES} ${Backtrace_LIBRARIES}")
   set(SWAY_Backtrace_HEADER "${Backtrace_HEADER}")
 endif()
 

common/CMakeLists.txt

@@ -1,5 +1,4 @@
 add_library(sway-common
-	${SWAY_Backtrace_HEADER}
 	ipc-client.c
 	list.c
 	log.c
@@ -7,3 +6,10 @@ add_library(sway-common
 	readline.c
 	stringop.c
 	)
+
+if(Backtrace_FOUND)
+	set_target_properties(sway-common
+		PROPERTIES
+		COMPILE_FLAGS "-include ${Backtrace_HEADER}"
+		)
+endif()