Trouble getting 256 mode working in latest Ubuntu

Hi there,

I recently upgraded to latest stable Ubuntu (18.04 LTS).
Unfortunately, after building screen from source, 256 mode no longer
seems to work. For example, if I launch vim, it reverts to the 8 color
color scheme.

This was a problem with both the 4.6.2 release (which appears to be
the latest officially tagged release) and the older 4.5.1 release that
I've been using on Ubuntu 16.04 and worked flawlessly with 256 colors.

I tried setting `term 'screen-256color'` inside ~/.screenrc, but that
caused errors with some terminal applications, for example when I ran
gdb:

$ gdb
"TERMCAP", line 20, col 1, terminal 'SC': Illegal character - '^J'
"TERMCAP", line 20, col 1, terminal 'SC': wrong type used for string
capability 'FA'
GNU gdb (GDB) 7.11.1