Fix appveyor

appveyor.yml

@@ -36,7 +36,7 @@ test_script:
   - if "%PLATFORM%" == "x64"
     call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x86_amd64
   - set PATH=%PATH%;%QTDIR%\bin
-  - qmake "DEFINES+=BOOST_INCLUDEDIR=C:\Libraries\boost_1_66_0" "DEFINES+=CGAL_DIR=%APPVEYOR_BUILD_FOLDER%\CGAL-4.11.1"
+  - qmake "BOOST_INCLUDEDIR=C:\Libraries\boost_1_66_0" "CGAL_DIR=%APPVEYOR_BUILD_FOLDER%\CGAL-4.11.1"
   - nmake -f Makefile.Release
     
 after_test:

dust3d.pro

@@ -86,9 +86,13 @@ SOURCES += src/main.cpp
 HEADERS += src/version.h
 
 win32 {
-	BOOST_INCLUDEDIR = $$(BOOST_INCLUDEDIR)
-	CGAL_DIR = $$(CGAL_DIR)
-
+	isEmpty(BOOST_INCLUDEDIR) {
+		BOOST_INCLUDEDIR = $$(BOOST_INCLUDEDIR)
+	}
+	isEmpty(CGAL_DIR) {
+		CGAL_DIR = $$(CGAL_DIR)
+	}
+	
 	isEmpty(BOOST_INCLUDEDIR) {
 		error("No BOOST_INCLUDEDIR define found in enviroment variables")
 	}