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This CL introduces 2 changes to the behavior of pkg_config when
use_sysroot=false:

1. The path is not made relative to the build directory.  This fixes the case
where your src directory may be mounted at different locations.

2. -I is used instead of -isystem.  When -isystem is used, dep files do not
reference the system header files.  This is OK when using the sysroot because
the CR_SYSROOT_HASH macro contains the sysroot revision so any source files will
be rebuilt when the sysroot updates.

R=dpranke

Change-Id: Ia3d7df51880205fee888923283b76078ccb3112f
Reviewed-on: https://chromium-review.googlesource.com/1159689Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580030}