cmake: support optional argument for overriding default ctag excludes

[tchaikov]

so instead of excluding all submodules, one can just exclude boost and
rocksdb using:

cmake -DCTAG_EXCLUDES="boost;rocksdb" ..

Signed-off-by: Kefu Chai kchai@redhat.com

[tchaikov]

the followon of #14334

[dmick]

Have you tested that the default run creates tag files are the same before and after? If so, I'm happy

[tchaikov]

ps shows

/bin/sh -c cd /var/ceph/ceph/src && /usr/bin/ctags -R --c++-kinds=+p --fields=+iaS --extra=+q "--exclude=*.js" --exclude=civetweb --exclude=erasure-code/jerasure/jerasure --exclude=erasure-code/jerasure/gf-complete --exclude=rocksdb --exclude=googletest --exclude=spdk --exclude=xxHash --exclude=isa-l --exclude=lua --exclude=Beast --exclude=boost --exclude=dpdk --exclude=zstd

and

$ ls -lh ../src/tags
-rw-r--r-- 1 kefu kefu 32M Apr  8 11:35 ../src/tags

also tested using vim.