Heads up: CLANG_CC, CLANG_CXX configuration variables renamed to LO_CLANG_CC, LO_CLANG_CXX

[Stephan Bergmann]

With
<https://gerrit.libreoffice.org/c/core/+/104383> "Rename CLANG_CC, 
CLANG_CXX configuration vars (avoid clash with scan-build)", those two 
variables (which can e.g. be passed into autogen.sh via autogen.input) 
are going to change their names.