Does not build with GCC 8

[Ankur Sinha]

The current version does not seem to build with GCC8. The downstream bug is here: https://bugzilla.redhat.com/show_bug.cgi?id=1582909

The build error is here:

/builddir/build/BUILD/lifeograph-1.4.2/src/diarydata.hpp:519:29: required from here
/usr/include/c++/8/bits/stl_tree.h:452:21: error: static assertion failed: comparison object must be invocable with two arguments of key type
       static_assert(__is_invocable<_Compare&, const _Key&, const _Key&>{},
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It appears to be a GCC8 change:
https://gcc.gnu.org/gcc-8/porting_to.html

[Ahmet Öztürk (dmxe)]

I am not sure if this isa problem in Lifeograph code or in GCC 8. Nevertheless, I've used a workaround to fix building with GCC8. The fix is in trunk.