Makefile: avoid unnecessary rebuilds of the library

[Lekensteyn]

• Do not wipe GOROOT for every _dev/go.sh invocation.
• Do not clone sidh and nobs repositories when only tris has changed.
• Do not copy and rebuild stdlib when only crypto/tls has changed.
• Avoid creating a temporary directory on every make invocation.
• Avoid creating src/src/ when the src/ directory already exists.

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Important fix for integration of Tris in external projects which use the _dev/go.sh wrapper. WIthout this fix, it would rebuild Tris for every single go invocation.

[Lekensteyn]

Changes since the initial PR:

• The bogo tests failed only on 1.12 because the bogo image was using 1.11 while the outside tests use 1.12.
• Moved another non-Linux build fix to PR Add ESNI client and server support #172 to avoid an overlap with that PR.
• ed4428f - ensure that stdlib is not considered complete until patch succeeds.
• 551e57e - ensure that $(GOROOT_LOCAL)/src is also wiped if Go was updated, otherwise files from old and new Go files might be mixed, resulting in the most bizarre error messages.

Initially the Makefile depended on $(GOROOT_LOCAL)/pkg under the assumption that this directory will have an older mtime whenever $(GOROOT_LOCAL)/pkg is updated. This was not the case with the bogo test and prevented the $(GOROOT_LOCAL)/src from being updated to the latest version. As a fix, now the version is included (as $(GOROOT_LOCAL)/pkg/.ok_$(VER_OS_ARCH).

[Lekensteyn]

Hi @henrydcase, any comments on this patch? It would remove unnecessary git clones during the build and speed up development.