configure can't find compiler although CC/CXX are given

1. Run configure.

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

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

   qtrob@rob-desktop:/tmp/qttest$ head -n3 config.summary 
   
   Building for: linux-g++ (x86_64, CPU features: mmx sse sse2)
   Compiler: gcc 9.3.0
   

   Fine!

3. Remove all files and subdirectories from the directory you configured in.
4. 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++
   

5. Run configure.

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

6. 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!

7. Remove all files and subdirectories from the directory you configured in.
8. Run configure with setting a platform.

   qtrob@rob-desktop:/tmp/qttest$ ~/dev/src/qtbase/configure -platform linux-clang
   

   Now configure can't find clang anymore:

   '/usr/bin/cmake' '-DQT_QMAKE_TARGET_MKSPEC=linux-clang' '-DCMAKE_C_COMPILER=clang' '-DCMAKE_CXX_COMPILER=clang++' '-G' 'Ninja' '/home/qtrob/dev/src/qtbase'
   -- The CXX compiler identification is unknown
   -- The C compiler identification is unknown
   -- The ASM compiler identification is unknown
   -- Didn't find assembler
   CMake Error at CMakeLists.txt:32 (project):
     The CMAKE_CXX_COMPILER:
   
       clang++
   
     is not a full path and was not found in the PATH.
   
     Tell CMake where to find the compiler by setting either the environment
     variable "CXX" or the CMake cache entry CMAKE_CXX_COMPILER to the full path
     to the compiler, or to the compiler name if it is in the PATH.
   
   
   CMake Error at CMakeLists.txt:32 (project):
     The CMAKE_C_COMPILER:
   
       clang
   
     is not a full path and was not found in the PATH.
   
     Tell CMake where to find the compiler by setting either the environment
     variable "CC" or the CMake cache entry CMAKE_C_COMPILER to the full path to
     the compiler, or to the compiler name if it is in the PATH.
   
   
   CMake Error at CMakeLists.txt:32 (project):
     No CMAKE_ASM_COMPILER could be found.
   
     Tell CMake where to find the compiler by setting either the environment
     variable "ASM" or the CMake cache entry CMAKE_ASM_COMPILER to the full path
     to the compiler, or to the compiler name if it is in the PATH.
   
   
   -- Warning: Did not find file Compiler/-ASM
   -- Configuring incomplete, errors occurred!
   See also "/tmp/qttest/CMakeFiles/CMakeOutput.log".
   See also "/tmp/qttest/CMakeFiles/CMakeError.log".
   CMake Error at /home/qtrob/dev/src/qtbase/cmake/QtProcessConfigureArgs.cmake:797 (message):
     CMake exited with code 1.
   

configure should always use the compilers from CC and CXX. Explicitly specifying such compilers and a platform should not lead into the situation that configure can't find any compiler.