write -isystem paths relative to the build root
write -isystem paths relative to the build root

The default makes these system-absolute paths,
which confuses fiddle's overlay builds.

Now we should see things like,

  -isystem ../third_party/externals/jsoncpp/include

where we previously had,

  -isystem /Users/mtklein/skia/third_party/externals/jsoncpp/include

Change-Id: I7b161a550fdb95b06e17c372cd5bec3015e3c8b7
Reviewed-on: https://skia-review.googlesource.com/109382
Reviewed-by: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>

google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,google/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,google/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,google/skia,google/skia,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,google/skia,rubenvb/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,google/skia,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,google/skia,rubenvb/skia,HalCanary/skia-hc,google/skia,Hikari-no-Tenshi/android_external_skia