Build: multiple GOPATH components

[djdv]

Version/Platform/Processor information (from ipfs version --all):
N/A
Type (bug, feature, meta, test failure, question):
Bug
Area (api, commands, daemon, fuse, etc):
Build system
Priority (from P0: functioning, to P4: operations on fire):
P1

Description:
A patch was merged #2808 that resolved this issue but was reverted #2862 there was no original issue ticket filed, I am filling this one so that the revert issue (#2833) may be closed without dropping awareness of the issue.

Original text by @karalabe from #2808 below:

Go's GOPATH environment variable can contain multiple paths, separated by a colon : similar to the PATH env var. In such cases Go uses the first path element to install any packages, however the remaining paths are still used to find dependencies. Because of this it's quite a common setup to have multiple paths listed in GOPATH (e.g. one for development and another for appengine, which has its special installer).

The current Makefile however erroneously assumes that GOPATH contains a single path element, and tries to locate go-ipfs within by appending a string to GOPATH, which is obviously cause an invalid path, and hence fail the path_check rule in the Makefile, even though the required condition is met.

[Kubuxu]

I think the problem is that in Windows the path separator would be ;.

So we would need if we run Windows && !Cygwin then change the path separator to ;.