libwidgets_essential - files are still compiled twice - warning settings lost

[Friedrich Beckmann]

Hi John,

i guess you wanted to avoid the double compilation of the „essential“ files for 
psppire and glade with this commit:

https://git.savannah.gnu.org/cgit/pspp.git/commit/?id=1834d3c0d3fa6307fbc0a8adff1eb9a77d997eed

But looking at the buildbot result this seems not work?

http://caeis.etech.fh-augsburg.de:8010/#/builders/4/builds/35

Further you have disabled the AM_CFLAGS warning settings for these files. When 
you revert

https://git.savannah.gnu.org/cgit/pspp.git/commit/?id=e15a8e2a3c12a29274d81c814e7e5058f0c65ad8

then this does not result in a warning for fallthrough in the case statement.

Fritz