Out of tree builds modifies in tree files

[magnusuMET]

• Version: v4.8.1, master
• Platform: Ubuntu 18.04, most likely other platforms

Description of the issue with the steps needed to reproduce it

• Have a platform with all build tools (including m4) included
• Build out of tree using CMake
• Running make modifies in-tree files (libsrc/{attr.c,ncx.c,putget.c})
• Running make clean removes in-tree files above and nc_test/{test_get.c,test_put.c,test_read.c,test_write.c}.

This is a problem when building a release from a read-only directory

[WardF]

Thanks, I'll sort this out.