Fix CMakeLists.txt regarding PythonQt.

CMakeLists.txt

@@ -80,8 +80,19 @@ IF(NOT CLIENT_ONLY)
     SET(PKG_CONFIG_EXTRA "qtversion=${Qt5Core_VERSION}")
   ENDIF(USE_QT4)
 
-  FIND_PACKAGE(PythonQt COMPONENTS QtAll)
-  SET(GEPETTO_GUI_HAS_PYTHONQT (${PythonQt_FOUND} AND ${PythonQt_QtAll_FOUND}) CACHE BOOL "Use PythonQt dependency")
+  # If GEPETTO_GUI_HAS_PYTHONQT is set to TRUE, then PythonQt is required.
+  IF(DEFINED GEPETTO_GUI_HAS_PYTHONQT AND GEPETTO_GUI_HAS_PYTHONQT)
+    FIND_PACKAGE(PythonQt REQUIRED COMPONENTS QtAll)
+  ELSE()
+    FIND_PACKAGE(PythonQt COMPONENTS QtAll)
+  ENDIF()
+  # Declare option GEPETTO_GUI_HAS_PYTHONQT
+  # If GEPETTO_GUI_HAS_PYTHONQT was defined, this does not change its value.
+  IF (PythonQt_FOUND AND PythonQt_QtAll_FOUND)
+    SET(GEPETTO_GUI_HAS_PYTHONQT TRUE CACHE BOOL "Use PythonQt dependency")
+  ELSE()
+    SET(GEPETTO_GUI_HAS_PYTHONQT FALSE CACHE BOOL "Use PythonQt dependency")
+  ENDIF()
 
   PKG_CONFIG_APPEND_LIBS(${PROJECT_NAME})