Relativize include dir for windows build

When we use relativized include dir, /showIncludes shows relativized
path for the files in include dir.

This is an effort of removing absolute path from compiling output.

Bug: 439949
Change-Id: I056cd8bb08c10f01855ef3bdb56ea1bcce1b769b
Reviewed-on: https://chromium-review.googlesource.com/1075947Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562599}

build/toolchain/win/setup_toolchain.py

@@ -240,6 +240,9 @@ def main():
       # The separator for INCLUDE here must match the one used in
       # _LoadToolchainEnv() above.
       include = [p.replace('"', r'\"') for p in env['INCLUDE'].split(';') if p]
+
+      # Make include path relative to builddir.
+      include = map(os.path.relpath, include)
       include_I = ' '.join(['"/I' + i + '"' for i in include])
       include_imsvc = ' '.join(['"-imsvc' + i + '"' for i in include])