tools/elfutils: remove header symlink between subdirectories

tools/elfutils: remove header symlink between subdirectories

Replace the symlink in the build recipe for gnulib
with an extra include path flag in CPPFLAGS to the lib subdirectory
so that it is the last in the order of include paths,
and use a quote escape to make the flag a literal string
in order to use Make variables within it.

The original reason this is necessary is because the config.h header
provided by the project at the top-level build directory
calls another header eu-config.h
which is stored in the lib subdirectory instead of the top-level,
and building the gnulib library requires the config.h header.

Signed-off-by: Michael Pratt <mcpratt@pm.me>
Link: https://github.com/openwrt/openwrt/pull/15853
Signed-off-by: Robert Marko <robimarko@gmail.com>