pkgTags  = map[string][]string{}    // map import dir to list of relevant tags
)

// tagKey returns the tag-based key to use in the pkgCache.
// It is a comma-separated string; the first part is dir, the rest tags.
// The satisfied tags are derived from context but only those that
// matter (the ones listed in the tags argument plus GOOS and GOARCH) are used.
// The tags list, which came from go/build's Package.AllTags,
// is known to be sorted.

		"-c",          // do not link
		"-xc",         // input language is C
	)
	if p.GccIsClang {
		c = append(c,
			"-ferror-limit=0",
			// Apple clang version 1.7 (tags/Apple/clang-77) (based on LLVM 2.9svn)
			// doesn't have -Wno-unneeded-internal-declaration, so we need yet another
			// flag to disable the warning. Yes, really good diagnostics, clang.
			"-Wno-unknown-warning-option",