Add a workaround for a glibc bug when -lphtread precedes -lgpgme

cgo_pthread_ordering_workaround.go

@@ -0,0 +1,32 @@
+package main
+
+/*
+This is a pretty horrible workaround.  Due to a glibc bug
+https://bugzilla.redhat.com/show_bug.cgi?id=1326903 , we must ensure we link
+with -lgpgme before -lpthread.  Such arguments come from various packages
+using cgo, and the ordering of these arguments is, with current (go tool link),
+dependent on the order in which the cgo-using packages are found in a
+breadth-first search following dependencies, starting from “main”.
+
+Thus, if
+   import "net"
+is processed before
+   import "…/skopeo/signature"
+it will, in the next level of the BFS, pull in "runtime/cgo" (a dependency of
+"net") before "mtrmac/gpgme" (a dependency of "…/skopeo/signature"), causing
+-lpthread (used by "runtime/cgo") to be used before -lgpgme.
+
+This might be possible to work around by careful import ordering, or by removing
+a direct dependency on "net", but that would be very fragile.
+
+So, until the above bug is fixed, add -lgpgme directly in the "main" package
+to ensure the needed build order.
+
+Unfortunately, this workaround needs to be applied at the top level of any user
+of "…/skopeo/signature"; it cannot be added to "…/skopeo/signature" itself,
+by that time this package is first processed by the linker, a -lpthread may
+already be queued and it would be too late.
+*/
+
+// #cgo LDFLAGS: -lgpgme
+import "C"