pkgconf: don't double prefix lib/include paths with sysroot

A .pc file could contain statements of the form:
    -L/usr/lib
but also:
    -L/path/to/sysroot/usr/lib

The latter form typically occurs when the dependency was configured with a
statement like --with-xxx=$(STAGING_DIR)/usr.

pkgconf only expects the first form, and prefixes it with the specified
sysroot. However, this strategy would result in a double sysroot in the
second case:
    -L/path/to/sysroot/path/to/sysroot/usr/lib

This patch checks for the presence of the sysroot in the specified -L or -I
directives, and only adds the sysroot when not already present.

Partially fixes bug #5750 (https://bugs.busybox.net/show_bug.cgi?id=5750

)

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>