When go parses #cgo lines, expand ${SRCDIR} into the path to the
source directory. This allows options to be passed to the
compiler and linker that involve file paths relative to the
source code directory. Without the expansion the paths would be
invalid when the current working directory changes.
Fixes #7891
Fixes #5428
Change-Id: I343a145a9771a5ccbaa958e4a1ecd1716fcae52d
Reviewed-on: https://go-review.googlesource.com/1756 Reviewed-by: Ian Lance Taylor <iant@golang.org>