Fix sed error in doxygen build on macOS
The macOS version of sed (perhaps any BSD-like sed?) produces this error message that causes the doyxgen build to fail:
/usr/bin/install -c -m 644 out/./bc_s.png '/Users/lpsinger/local/share/doc/lalmetaio/./bc_s.png'
sed: RE error: illegal byte sequence
This is because the png contains a byte sequence that is not
valid UTF-8. Fix this by setting LC_ALL=C
when calling sed.