Platform: properly link X11 to all applications that need it.

Discovered by the fresh Platform tests -- the X11 package was called
*after* X11_LIBRARIES was used.

src/Magnum/Platform/CMakeLists.txt

@@ -231,6 +231,11 @@ if(WITH_GLXAPPLICATION)
     set(NEED_GLXCONTEXTHANDLER 1)
     set(NEED_GLXCONTEXT 1)
 
+    find_package(X11)
+    if(NOT X11_FOUND)
+        message(FATAL_ERROR "X11 library, required by some applications, was not found. Set WITH_*X*APPLICATION to OFF to skip building them.")
+    endif()
+
     set(MagnumGlxApplication_SRCS
         GlxApplication.cpp
         $<TARGET_OBJECTS:MagnumAbstractXApplication>
@@ -269,6 +274,11 @@ if(WITH_XEGLAPPLICATION)
         message(FATAL_ERROR "EGL library, required by some applications, was not found. Set WITH_*EGL*APPLICATION to OFF to skip building them.")
     endif()
 
+    find_package(X11)
+    if(NOT X11_FOUND)
+        message(FATAL_ERROR "X11 library, required by some applications, was not found. Set WITH_*X*APPLICATION to OFF to skip building them.")
+    endif()
+
     set(MagnumXEglApplication_SRCS
         XEglApplication.cpp
         $<TARGET_OBJECTS:MagnumAbstractXApplication>
@@ -297,13 +307,6 @@ if(WITH_XEGLAPPLICATION)
     add_library(Magnum::XEglApplication ALIAS MagnumXEglApplication)
 endif()
 
-if(WITH_WINDOWLESSGLXAPPLICATION OR NEED_ABSTRACTXAPPLICATION)
-    find_package(X11)
-    if(NOT X11_FOUND)
-        message(FATAL_ERROR "X11 library, required by some contexts, was not found. Set WITH_*X*APPLICATION to OFF to skip building them.")
-    endif()
-endif()
-
 # Windowless EGL application
 if(WITH_WINDOWLESSEGLAPPLICATION)
     set(NEED_EGLCONTEXT 1)
@@ -343,6 +346,11 @@ endif()
 if(WITH_WINDOWLESSGLXAPPLICATION)
     set(NEED_GLXCONTEXT 1)
 
+    find_package(X11)
+    if(NOT X11_FOUND)
+        message(FATAL_ERROR "X11 library, required by some applications, was not found. Set WITH_*X*APPLICATION to OFF to skip building them.")
+    endif()
+
     set(MagnumWindowlessGlxApplication_SRCS
         WindowlessGlxApplication.cpp
         $<TARGET_OBJECTS:MagnumGlxContextObjects>)