Fix FC_DEFAULT_FONTS on macOS and with BSD sed

Fix processing of FC_DEFAULT_FONTS when it contains multiple comma-
separated values (i.e. on macOS/Darwin) and fix inserting it into
fonts.conf when using BSD sed.

Fixes fontconfig/fontconfig#287
11 jobs for ryandesign-FC_DEFAULT_FONTS in 9 minutes and 27 seconds (queued for 14 seconds)
latest
Status Job ID Name Coverage
  Build
passed meson android arm64 fedora #11642103

00:01:41

passed meson macos #11642101
gst-macos-10.15

00:01:33

passed meson-mingw-w64-build #11642098

00:04:56

manual meson msys2 #11642102
docker windows 1809 allowed to fail manual
passed meson-shared-build #11642096

00:04:09

passed meson-static-build #11642097

00:05:19

passed meson vs2017 amd64 #11642099
docker windows 1809

00:01:18

passed meson vs2017 x86 #11642100
docker windows 1809

00:01:16

passed mingw-build #11642095

00:08:10

passed shared-build #11642093

00:09:27

passed static-build #11642094

00:04:27