Locate packcc in build directory instead of the source

[Myz]

This should fix a Out-of-source or nested build.

Related to #2029.

[coveralls]

Coverage remained the same at 85.237% when pulling 5de4826 on Myz:master into affa2f7 on universal-ctags:master.

[b4n]

Looks reasonable. We should have an out-of-tree build on CI I guess, it's always nice to catch these kind of things early.

[masatake]

NOTE: out-of-tree build can be done with cd $(mktemp -d) && git clone https://github.com/universal-ctags/ctags.git && cd ctags && ./autogen.sh && mkdir mybuild && cd mybuild && ../configure && make -j10 as @Myz written in #2029.

[masatake]

Thank you!

[Myz]

Anytime