cmake: only build GPU library if CUDA is present

lib/CMakeLists.txt

 add_subdirectory(common)
-add_subdirectory(gpu)
 
 set(SOURCES
 	vlnv.cpp
@@ -36,7 +35,16 @@ find_package(Threads)
 add_library(villas-fpga SHARED ${SOURCES})
 
 target_link_libraries(villas-fpga PUBLIC villas-common)
-target_link_libraries(villas-fpga PUBLIC villas-gpu)
+
+# GPU library is optional, check for CUDA presence
+include(CheckLanguage)
+check_language(CUDA)
+if(CMAKE_CUDA_COMPILER)
+	add_subdirectory(gpu)
+	target_link_libraries(villas-fpga PUBLIC villas-gpu)
+else()
+	message("No CUDA support, not building GPU library")
+endif()
 
 target_compile_definitions(villas-fpga PRIVATE
 	BUILDID=\"abc\"

tests/CMakeLists.txt

@@ -4,7 +4,6 @@ set(SOURCES
 	logging.cpp
 	dma.cpp
     fifo.cpp
-	gpu.cpp
 #	hls.c
 #	intc.c
 #	rtds_rtt.c
@@ -13,6 +12,10 @@ set(SOURCES
     graph.cpp
 )
 
+if(CMAKE_CUDA_COMPILER)
+	list(APPEND SOURCES gpu.cpp)
+endif()
+
 add_executable(unit-tests ${SOURCES})
 
 find_package(Criterion REQUIRED)