Allow building against system libspng

CMakeLists.txt

@@ -800,7 +800,15 @@ else()
   set(LZO lzo2)
 endif()
 
+pkg_check_modules(pc_spng IMPORTED_TARGET spng)
+if (pc_spng_FOUND AND TARGET PkgConfig::pc_spng)
+  message(STATUS "Using the system libspng")
+  set(spng_target PkgConfig::pc_spng)
+else()
+  message(STATUS "Using static libspng from Externals")
   add_subdirectory(Externals/libspng)
+  set(spng_target spng)
+endif()
 
 # Using static FreeSurround from Externals
 # There is no system FreeSurround library.

Source/Core/Common/CMakeLists.txt

@@ -155,7 +155,7 @@ PRIVATE
   ${CURL_LIBRARIES}
   FatFs
   ${ICONV_LIBRARIES}
-  spng
+  ${spng_target}
   ${VTUNE_LIBRARIES}
 )
 

Source/Core/InputCommon/CMakeLists.txt

@@ -81,7 +81,7 @@ PUBLIC
 
 PRIVATE
   fmt::fmt
-  spng
+  ${spng_target}
 )
 
 if(WIN32)

Source/Core/VideoCommon/CMakeLists.txt

@@ -170,7 +170,7 @@ PUBLIC
   core
 PRIVATE
   fmt::fmt
-  spng
+  ${spng_target}
   xxhash
   imgui
   implot