Tips for compiling LEMON with gcc-9?

Hello,

The lemon 1.3.1 source package for Debian no longer builds as Debian has upgraded to GCC 9.

Here is the original report:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=925749

Adding '-std=c++2a' to the CXXFLAGS makes the compilation succeed, but then one of the tests fail:

26: Test command: /build/liblemon-1.3.1+dfsg/obj-x86_64-linux-gnu/test/lgf_reader_writer_test
26: Test timeout computed to be: 10000000
26: terminate called after throwing an instance of 'lemon::FormatError'
26: what(): lemon:FormatError: Item not found
26/44 Test #26: lgf_reader_writer_test ...........Child aborted***Exception: 0.17 sec

I also tested the latest code from the HG repository at http://lemon.cs.elte.hu/hg/lemon/archive/ed2c21cbd6ef.tar.bz2 (corresponding to the tip of the 1.3 branch) and had the same results. While it doesn't need '-std=c++2a'in CXXFLAGS, it still has the same test failure as above.

Any hints for getting LEMON to compile with gcc-9 are very appreciated!

Michael R. Crusoe