CMake: Warn if empty QTDIR/DepsPath vars on Windows

Closes jp9000/obs-studio#608

CMakeLists.txt

@@ -2,6 +2,21 @@ cmake_minimum_required(VERSION 2.8.12)
 
 project(obs-studio)
 
+if(WIN32)
+	if (QTDIR OR DEFINED ENV{QTDIR} OR DEFINED ENV{QTDIR32} OR DEFINED ENV{QTDIR64})
+		# Qt path set by user or env var
+	else()
+		set(QTDIR "" CACHE PATH "Path to Qt (e.g. C:/Qt/5.7/msvc2015_64)")
+		message(WARNING "QTDIR variable is missing.  Please set this variable to specify path to Qt (e.g. C:/Qt/5.7/msvc2015_64)")
+	endif()
+	if (DepsPath OR DEFINED ENV{DepsPath} OR DEFINED ENV{DepsPath32} OR DEFINED ENV{DepsPath64})
+		# Dependencies path set by user or env var
+	else()
+		set(DepsPath "" CACHE PATH "Path to compiled dependencies (e.g. D:/dependencies/win64)")
+		message(WARNING "DepsPath variable is missing.  Please set this variable to specify path to compiled dependencies (e.g. D:/dependencies/win64)")
+	endif()
+endif()
+
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")
 
 include(ObsHelpers)