At least for me on Linux since LO 5.3, 'soffice
sw/qa/extras/rtfexport/data/fdo72031.rtf' shows "Å" (rendered in "DejaVu Sans")
instead of "⊕" (rendered in "Standard Symbols L").  That's presumably because
47ea13ef "Kill the old Unix layout engines"
removed support for Type 1 fonts (see "Ignore Type 1 fonts" in
FontCfgWrapper::addFontSet, vcl/unx/generic/fontmanager/fontconfig.cxx), and my
(Fedora 25) /usr/share/fonts/default/Type1/s050000l.pfb "Standard Symbols L" is
a Type 1 font.  So we fell back to fontconfig's generic (weak) suggestion of
"DejaVu Sans" as a substitute for "Symbol".

So extend our fc_local.conf to suggest our "OpenSymbol" as a substitute for
"Symbol".

As that fc_local.conf was originally brought along by --with-fonts, which is
enabled by default but can be disabled, compilation of fc_local.conf from the
various snippets is moved to postprocess.

macOS and Windows were never affected, as they both come with a "Symbol" font
installed in the system.  (And we don't install fc_local.conf on Windows at
all.)

Change-Id: I8d6d87f24974577fd66f5f3989f606237ebb3d75
Reviewed-on: https://gerrit.libreoffice.org/42670Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>