Log Message:
lilypond: fix builds with GNU grep >= 3.5

As of 3.5, GNU grep changed so by default, "The message that a binary
file matches is now sent to standard error", which broke the
expectations of the configure script, which was calling grep on font
files without using -a or equivalent options. If we change to use -a,
this will break non-GNU grep implementations that don't have it, so
also universally require GNU grep for consistency/simplicity.

We should be fine here, as -a was added to GNU grep 2.4.1 back twenty
years ago and pkgsrc sets 2.5.1 as its minimum tool version if building
it itself.

(Really this package should be updated to a newer stable release branch
but I don't have time right now.)