[build] Don't fail when passed --disable-lcov and lcov isn't available
Summary:
Backport of core PR11611
https://github.com/bitcoin/bitcoin/pull/11611/files
Test Plan:
sudo apt remove lcov ./autogen.sh mkdir build && cd build ../configure --disable-lcov
Check that configure does not fail.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3546