CMake: Disable VST (GTK) when gdk-x11-3.0 isn't found [wayland]

The VST plugin done via GTK for non-apple & non-windows pulls <gdk/gdkx.h>,
which is specific to X11 and thus isn't available in a pure-wayland system.

This disables it automatically.

Closes: https://github.com/tenacityteam/tenacity/issues/614
Signed-off-by: Haelwenn (lanodan) Monnier <contact@hacktivis.me>

CMakeLists.txt

@@ -798,6 +798,13 @@ endif()
 if( NOT CMAKE_SYSTEM_NAME MATCHES "Darwin|Windows" )
   find_package( GLIB REQUIRED )
   find_package( GTK 3.0 REQUIRED )
+  pkg_check_modules(GDK3_X11 gdk-x11-3.0)
+endif()
+
+if( NOT CMAKE_SYSTEM_NAME MATCHES "Darwin|Windows" )
+  cmake_dependent_option( USE_VST "Use VST2 plug-in support [ON, OFF]" ON "GDK3_X11_FOUND" OFF)
+else()
+  cmd_option( USE_VST "Use VST2 plug-in support [ON, OFF]" ON)
 endif()
 
 if( NOT WIN32 )

src/CMakeLists.txt

@@ -57,10 +57,6 @@ endif()
 cmd_option( use_ladspa "Use LADSPA plug-in support [ON, OFF]" ON )
 set( USE_LADSPA ${use_ladspa} CACHE INTERNAL "" )
 
-# Handle VST option
-cmd_option( use_vst "Use VST2 plug-in support [ON, OFF]" ON )
-set( USE_VST ${use_vst} CACHE INTERNAL "" )
-
 # ~~~
 # General source files
 # ~~~