Improve ccache setup in CMakeLists.txt
Improve ccache setup in CMakeLists.txt

CMAKE_IGNORE_PATH is empty by default, so we should not ignore
it if set by the user, since that's a surprising behavior.

http://www.catb.org/esr/writings/taoup/html/ch01s06.html#id2878339

karies/root,karies/root,karies/root,olifre/root,karies/root,root-mirror/root,karies/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root