libClangFormat build fails without clang static libs

Commit 6cff79d377b4dc5e914873f008d3c6f1ec74ddf2 allows building qtcreator when only shared clang libs are available, but the ClangFormat plugin still has a hardcoded dependency on clangFormat, which breaks build if the static library is not present:

[ 76%] Linking CXX shared library ../../../lib/qtcreator/plugins/libClangFormat.so

/usr/bin/ld: cannot find -lclangFormat