Proposing to make vulkan support eventually optional.

Still some alternative oses does not have vulkan support or not full (e.g. Raspberry).

CMakeLists.txt

@@ -47,6 +47,7 @@ string(REPLACE "-O3" "-O2" CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE}")
 # Compilation time options.
 option(CURL_SUPPORT "cURL support" ON)
 option(OPENAL_SUPPORT "OpenAL support" ON)
+option(VULKAN_SUPPORT "Vulkan support" ON)
 option(SYSTEMWIDE_SUPPORT "Enable systemwide installation of game assets" OFF)
 
 # These variables will act as our list of include folders and linker flags.
@@ -122,10 +123,12 @@ else()
 	add_definitions(-DHAVE_EXECINFO)
 endif()
 
+if (${VULKAN_SUPPORT})
     # We need an Vulkan implementation.
     find_package(Vulkan REQUIRED)
     list(APPEND yquake2IncludeDirectories ${Vulkan_INCLUDE_DIR})
     list(APPEND yquake2VulkanLinkerFlags ${Vulkan_LIBRARIES})
+endif()
 
 # cURL support.
 if (${CURL_SUPPORT})
@@ -576,6 +579,7 @@ set(SOFT-Header
 	${COMMON_SRC_DIR}/header/shared.h
 	)
 
+if (${VULKAN_SUPPORT})
     set(VK-Source
 	${REF_SRC_DIR}/vk/vk_buffer.c
 	${REF_SRC_DIR}/vk/vk_cmd.c
@@ -615,6 +619,7 @@ set(VK-Header
 	${REF_SRC_DIR}/vk/header/util.h
 	${COMMON_SRC_DIR}/header/shared.h
 	)
+endif()
 
 # Wrapper for the Windows binary
 if(${CMAKE_SYSTEM_NAME} MATCHES "Windows")
@@ -703,6 +708,7 @@ set_target_properties(ref_soft PROPERTIES
 		)
 target_link_libraries(ref_soft ${yquake2LinkerFlags} ${yquake2SDLLinkerFlags})
 
+if(${VULKAN_SUPPORT})
     # Build the vk renderer dynamic library
     add_library(ref_vk MODULE ${VK-Source} ${VK-Header} ${REF-Platform-Specific-Source})
     set_target_properties(ref_vk PROPERTIES
@@ -712,3 +718,4 @@ set_target_properties(ref_vk PROPERTIES
 		SUFFIX ${CMAKE_SHARED_LIBRARY_SUFFIX}
 		)
     target_link_libraries(ref_vk ${yquake2LinkerFlags} ${yquake2VulkanLinkerFlags} ${yquake2SDLLinkerFlags})
+endif()