Even if we don't use swig in our go projects, if hosting environment has
swig, but incorrectly installed, then Go build will fail as:

   --- FAIL: TestScript (0.03s)
       --- FAIL: TestScript/list_swigcxx (0.46s)
           script_test.go:252:
               # go list should not report SWIG-generated C++ files in CompiledGoFiles. (0.001s)
               # CompiledGoFiles should contain 4 files:
               #  a.go
               #  a.swigcxx.go
               #  _cgo_gotypes.go
               #  a.cgo1.go (0.421s)
               > go list -f '{{.CompiledGoFiles}}' -compiled=true example/swig
               [stdout]
               []
               [stderr]
               # example/swig
               :1: Error: Unable to find 'swig.swg'
               :3: Error: Unable to find 'go.swg'
               [exit status 2]
               FAIL: testdata/script/list_swigcxx.txt:12: unexpected command failure

Such broken environment, in particular, is present on our testnodes,
because there swig program is being included into slapos-node package,
slapos command includes it into $PATH for spawned programs

    https://lab.nexedi.com/nexedi/slapos/blob/99cf4bfd/component/slapos/buildout.cfg#L22-30
    https://lab.nexedi.com/nexedi/slapos/blob/99cf4bfd/component/slapos/buildout.cfg#L74-88

but the swig binary itself is configured to look into its supporting files in
the wrong place:

    slapuser91@vifibcloud-rapidspace-hosting-007:~/t/swig$ /opt/slapos/parts/swig/bin/swig -swiglib
    /usr/src/packages/BUILD/slapos/build/opt/slapos/parts/swig/share/swig/3.0.10

    slapuser91@vifibcloud-rapidspace-hosting-007:~/t/swig$ ll /usr/src/packages/BUILD/slapos/build/opt/slapos/parts/swig/share/swig/3.0.10
    ls: cannot access '/usr/src/packages/BUILD/slapos/build/opt/slapos/parts/swig/share/swig/3.0.10': No such file or directory

which leads to SWIG being broken:

    slapuser91@vifibcloud-rapidspace-hosting-007:~/t/swig$ /opt/slapos/parts/swig/bin/swig -v -go -intgosize 64 a.swigcxx
    Language subdirectory: go
    Search paths:
       ./
       ./swig_lib/go/
       /usr/src/packages/BUILD/slapos/build/opt/slapos/parts/swig/share/swig/3.0.10/go/
       ./swig_lib/
       /usr/src/packages/BUILD/slapos/build/opt/slapos/parts/swig/share/swig/3.0.10/
    Preprocessing...
    :1: Error: Unable to find 'swig.swg'
    :3: Error: Unable to find 'go.swg'

-> Fix it by building SWIG ourselves and using that built swig to build go and
in the Go environment provided to users.

See "Troubleshooting" in http://www.swig.org/Release/README for details.

/cc @luke, @jerome, @romain