OOo used to use debug level this way:
0 is default for product build
1 for --enable-dbgutil
2 for environment variable debug=t, regardless of dbgutil setting
in LO these have now morphed into:
0 is default for product build
1 for environment variable debug=t
2 for --enable-dbgutil (at some intermediate stage in the past)
1 for --enable-dbgutil (today)

This has caused a lot of confusion and some things were not converted
properly, including a check in LinkTarget.mk.

Developers should use --enable-debug to get useful information,
including full debug info and assertions.

--enable-symbols is not intended for developers, but for people
who need their builds to be able to produce useful stack traces;
--enable-symbols is for example set unconditionally in the Fedora RPM
spec, and for this purpose disabling of inlining is not wanted.
It is unlikely that somebody wants symbols for only part of the build,
and consequently a different mechanism is used to set the corresponding
flags: they are set into global gb_LinkTarget_C{,XX}FLAGS.

So move the selective symbol feature back to --enable-debug.

(this substantially reverts db8df57a)