compiler given by CC/CXX is ignored by qmake

1. Set environment variables for using clang compilers:

   qtrob@rob-desktop:/tmp/qttest$ export CC=/usr/lib/llvm-10/bin/clang
   qtrob@rob-desktop:/tmp/qttest$ export CXX=/usr/lib/llvm-10/bin/clang++
   

   No, they are not in PATH.

2. Run configure.

   qtrob@rob-desktop:/tmp/qttest$ ~/dev/src/qtbase/configure  -prefix $PWD
   

   It will finish with:

   Once everything is built, Qt is installed. You should NOT run 'cmake --install .'
   

3. Check config.summary for the automatically detected compiler:

   qtrob@rob-desktop:/tmp/qttest$ head -n3 config.summary 
   
   Building for: linux-clang (x86_64, CPU features: mmx sse sse2)
   Compiler: clang 10.0.0
   

   Fine!

4. Build:

   cmake --build . --parallel
   

   Everything seems good this far.

5. Try using qmake from this build:

   qtrob@rob-desktop:/tmp/qttest$ cd /tmp/result/
   qtrob@rob-desktop:/tmp/result$ ls -a
   .  ..
   qtrob@rob-desktop:/tmp/result$ /tmp/qttest/bin/qmake ~/dev/src/qtbase/examples/gui/analogclock/analogclock.pro
   Project ERROR: Cannot run compiler 'clang++'. Output:
   ===================
   ===================
   Maybe you forgot to setup the environment?
   

   Yes, CC and CXX are still set at this time.

All tools from a Qt build should be usable, also when CC/CXX were used. When Qt was configured to use a certain compiler, qmake should use this.