Q:

Go 1.5+ : Error - imports runtime: C source files not allowed when not using cgo or SWIG

My program was building perfectly with go 1.4.2.  After installing go 1.5, I am getting following error. 

imports runtime: C source files not allowed when not using cgo or
  SWIG: atomic_amd64x.c defs.c float.c heapdump.c lfstack.c malloc.c
  mcache.c mcentral.c mem_linux.c mfixalloc.c mgc0.c mheap.c msize.c
  os_linux.c panic.c parfor.c proc.c runtime.c signal.c signal_amd64x.c
  signal_unix.c stack.c string.c sys_x86.c vdso_linux_amd64.c

The error is not program dependent.  Even a "Hello world" programs is throwing the same error. 

A:

Remove Old 1.4.x go installation and then install go 1.5. 

I faced the same problem after installing go 1.5.  The issue is that I installed go 1.5 at the same place of 1.4.2.  Go 1.5 does not use C anymore and if old C installation files are present it throws the error as you are shown. 
if you want the old installation to be present as well then install 1.5 to some other location and set GOROOT to that location. 
Hope it helps. 
Edit 1: For Linux Users:
if you downloaded go1.5*.tar.gz then you did this: 

tar -C /usr/local/ -xvf go1.5.linux-amd64.tar.gz

Your old folder C files are not deleted, so the correct process is: 
First: You should delete you old go folder. 

sudo rm -rf /usr/local/go/

Second: Then run  

tar -C /usr/local/ -xvf go1.5.x.linux-amd64.tar.gz

A:

If you use brew to upgrade your go to Go 1.5.   Here is my step to make it work:

Remove pkg folder in %GOPATH%
Reload terminal(console) session, make sure Go env goes to Go 1.5 folder. 

BTW: You need do the same thing if you want to switch from Go 1.5 back Go 1.4.2 via brew switch go 1.4.2.