pkgconf: always retain -I and -L flags

The pkgconf fork filters -I and -L flag values from .pc files which match
pkgconf's builtin system directory value.

During configure, pkgconf derives the default system include and library
search path values from exec_prefix, which is set to staging_dir/host in
the host tool build phase.

Due to that, pkgconf will drop all -I and -L flags pointing to
staging_dir/host/include or staging_dir/host/lib, unless invoked with
--keep-system-cflags and --keep-system-libs respectively, breaking our
kernel libelf discovery / stack validation workarounds.

In order to inhibit the filtering, add --keep-system-cflags and
--keep-system-libs to our pkg-config shell wrapper.

Fixes: GH#2832
Fixes: 867298cf47 ("tools/pkg-config: Replace with pkgconf")
Ref: https://lists.infradead.org/pipermail/openwrt-devel/2020-March/022182.html
Ref: https://git.openwrt.org/fe43969336201f2cc7d103b68fd6e65989bee184
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Acked-by: Petr Štetiar <ynezz@true.cz>

tools/pkgconf/files/pkg-config

@@ -1,6 +1,8 @@
 #!/bin/sh
 
 pkg-config.real \
+--keep-system-cflags \
+--keep-system-libs \
 --define-variable=prefix="${STAGING_PREFIX}" \
 --define-variable=exec_prefix="${STAGING_PREFIX}" \
 --define-variable=bindir="${STAGING_PREFIX}/bin" "$@"