configure fails when BUILDDIR path contains "++"

• GIVEN: a qt5.git checkout in some $SRCDIR (doesn't matter)
• WHEN: trying to run its top-level configure in a $BUILDDIR that ends in qt5-build-c++17

  $SRCDIR/configure -prefix $PWD
  

• THEN: configure fails with the following error:

  CMake Error at qtwebengine/src/CMakeLists.txt:152 (if):
    if given arguments:
  
      "NOT" "Gn_FOUND" "OR" "Gn_EXECUTABLE" "MATCHES" "^/home/marc/Qt/qt5-build-c++17/qtwebengine/install"
  
    Regular expression "^/home/marc/Qt/qt5-build-c++17/qtwebengine/install"
    cannot compile
  

Expected: Paths are escaped before being used as regular expressions.