Check that qmake runs successfully and check Qt version

qrenderdoc/CMakeLists.txt

@@ -8,6 +8,22 @@ endif()
 set(QMAKE_QT5_COMMAND ${QT_QMAKE_EXECUTABLE} CACHE STRING "Command to run to invoke Qt5's qmake. Normally this is qmake, possibly with qtchooser, but might be qmake-qt5")
 set(RENDERDOC_SWIG_PACKAGE https://github.com/baldurk/swig/archive/renderdoc-modified-7.zip CACHE STRING "The location where RenderDoc's swig fork source can be found. By default points to the URL on github but can be pointed to a local file.")
 
+execute_process(
+    COMMAND ${QT_QMAKE_EXECUTABLE} -query QT_VERSION
+    WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+    OUTPUT_VARIABLE QT_VERSION
+    ERROR_VARIABLE QT_VERSION_STDERR
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    ERROR_STRIP_TRAILING_WHITESPACE
+    )
+
+# we need the trailing .999 since 5.5.1 would be VERSION_GREATER than 5.5 which implicitly means 5.5.0
+if (QT_VERSION VERSION_GREATER 5.5.999)
+    message(STATUS "Building using Qt ${QT_VERSION}")
+else()
+    message(FATAL_ERROR "${QT_QMAKE_EXECUTABLE} -query QT_VERSION returned: \"${QT_VERSION}\". Error output was: \"${QT_VERSION_STDERR}\". We require Qt >= 5.6 and for the qmake executable to run Qt5. Check your distribution for how to configure Qt5, you may need to set the QT_SELECT environment variable or customise the cmake variable QT_QMAKE_EXECUTABLE to qmake-qt5")
+endif()
+
 set(QMAKE_CONFIG "debug")
 set(QMAKE_LDFLAGS "")
 set(QMAKE_CXXFLAGS "")