Error generating coverage report

I'm working on improving coverage and would like to generate some
reports (other than the text versions) to help me find uncovered code.
However, my source path and build path are not the same and I'm running
into problems.

This works fine and produces gcov output:

$ make -C test/build/src/bin/pg_basebackup check
$ ls -1 test/build/src/bin/pg_basebackup/*.gcno

test/build/src/bin/pg_basebackup/pg_basebackup.gcno
test/build/src/bin/pg_basebackup/pg_receivewal.gcno
test/build/src/bin/pg_basebackup/pg_recvlogical.gcno
test/build/src/bin/pg_basebackup/receivelog.gcno
test/build/src/bin/pg_basebackup/streamutil.gcno
test/build/src/bin/pg_basebackup/walmethods.gcno

But, when I try to generate the coverage report I get:

$ make -C test/build/src/bin/pg_basebackup coverage-html

make: Entering directory `/home/vagrant/test/build/src/bin/pg_basebackup'
/usr/bin/lcov --gcov-tool /usr/bin/gcov -q --no-external -c -i -d . -d
/postgres/src/bin/pg_basebackup -o lcov_base.info
geninfo: ERROR: no .gcno files found in /postgres/src/bin/pg_basebackup!
make: *** [lcov_base.info] Error 255
make: *** Deleting file `lcov_base.info'
make: Leaving directory `/home/vagrant/test/build/src/bin/pg_basebackup'

It's looking at my source directory rather than my build directory.
Here's how I'm calling configure:

$ cd /home/vagrant/test/build
$ /postgres/configure --prefix=/home/vagrant/test/pg \
--docdir=/postgres-doc --enable-cassert --enable-tap-tests \
--enable-coverage --with-openssl --enable-depend

I read docs/10/static/regress-coverage.html but didn't see anything that
helps. Hopefully I'm missing something?

Thanks,
--
-David
david(at)pgmasters(dot)net