runtime/cgo: report a meaningful error message when using Cygwin
Go has never supported Cygwin as a C compiler, but users get the
following cryptic error message when they try to use it:
implicit declaration of function '_beginthread'
This is because Cygwin doesn't implement _beginthread. Note that
this is not the only problem with Cygwin, but it's the one that
users are most likely to run into first.
This CL improves the error message to make it clear that Cygwin
is not supported, and suggests using MinGW instead.
Fixes #59490
Fixes #36691
Change-Id: Ifeec7a2cb38d7c5f50d6362c95504f72818c6a76
Reviewed-on: https://go-review.googlesource.com/c/go/+/627935
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>