exim: do not link buildconfig with $(LIBS)

exim: do not link buildconfig with $(LIBS)

The existing patch 0001-Build-buildconfig-for-the-host.patch changes
the exim build system to use the host compiler to build the
"buildconfig" program instead of the cross-compiler.

However, it still uses $(LIBS) which lists the target libraries to
link with, which shouldn't be used. Since buildconfig doesn't use any
library beyond the C library, we can simply drop using $(LIBS).

This will fix build failures of exim on Fedora 28, where libnsl is no
longer provided by the C library, causing build failures such as:

/usr/bin/gcc buildconfig.c
/usr/bin/ld: cannot find -lnsl

Fixes:

  http://autobuild.buildroot.net/results/ac78fe18657558b3c12c03c08bf1081d7c06ca85/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>