...that is documented as: "Does nothing.  Preserved for backward compatibility."
ever since <https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=160384> from
2010.

-fno-default-inline was the only value ever set in gb_DEBUG_CXXFLAGS, so the
latter can be removed now.

The use of gb_DEBUG_CXXFLAGS had accidentally already been removed from
gb_LinkTarget__get_debugcxxflags with e751e242
"--enable-optimized should be orthogonal to --enable-debug/--enable-dbgutil",
and that leaves gb_LinkTarget__get_debugcflags and
gb_LinkTarget__get_debugcxxflags with identical definitions, so replace those
two with a single gb_LinkTarget__get_debugflags.

Some external modules had used only gb_DEBUG_CXXFLAGS, when this was apparently
meant to be used in addition to gb_DEBUG_CFLAGS, so those uses have been changed
to gb_DEBUG_CFLAGS now.

Change-Id: I84ea0ab1233569b0b02ca057240a71f138352381
Reviewed-on: https://gerrit.libreoffice.org/66808
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>