Issue 5648: Allow user to provide CXXFLAGS on command line

https://sourceforge.net/p/testlilyissues/issues/5648/

I've found these changes helpful while experimentally enabling some
optional warnings.  I'm submitting these on their own because I'm not
sure how much longer I'll need to finish up the related warning work.

1: move -Woverloaded-virtual from CXXFLAGS to EXTRA_CXXFLAGS

2: move CXXFLAGS to the end of ALL_CXXFLAGS

This allows CXXFLAGS=... on the command line to override flags in
ALL_CXXFLAGS.

3: Stop copying FlexLexer.h

Leave FlexLexer.h where it is, rather than copying it to $(outdir).

As we enable more warnings, we will not want to hear about problems in
this file.  g++ automatically suppresses warnings for headers found in
system include directories (specified with -isystem), so the simple
solution is to leave it where it is.

[lemzwerg, 2019-12-26 09:08:38]

LGTM, thanks!