config: put CFLAGS/LDLIBS in config.mk

config: put CFLAGS/LDLIBS in config.mk

This renames Config to config.mk and includes more Make input.
Now configure generates all the required CFLAGS and LDLIBS for
the optional libraries.

Also, use pkg-config to test for libelf, rather than using a test
program. This makes it consistent with other libraries.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>