cmake: Fix find_program usage for 3.15

yuzu requires CMake 3.15 yet find_program was using REQUIRED, which is
only available on 3.18 and later. Instead, we check for
"<VAR>-NOTFOUND".

In addition, check for additional requirements before building libusb or
FFmpeg with autotools. Otherwise, CMake configuration will pass yet
compilation will fail.

src/video_core/host_shaders/CMakeLists.txt

@@ -18,7 +18,10 @@ set(SHADER_FILES
     vulkan_uint8.comp
 )
 
-find_program(GLSLANGVALIDATOR "glslangValidator" REQUIRED)
+find_program(GLSLANGVALIDATOR "glslangValidator")
+if ("${GLSLANGVALIDATOR}" STREQUAL "GLSLANGVALIDATOR-NOTFOUND")
+    message(FATAL_ERROR "Required program `glslangValidator` not found.")
+endif()
 
 set(GLSL_FLAGS "")
 set(QUIET_FLAG "--quiet")