Build inherits $SRC from shell

[seanmcl]

CBMC version: develop branch
Operating system: OSX
Exact command line resulting in the issue: make -C src
What behaviour did you expect: build
What happened instead: confusing, unrelated error message about something (the value of SRC) being a directory.

[seanmcl]

To reproduce

$ git clone cbmc-url
$ SRC=/var make -C src
## Entering big-int
/Applications/Xcode.app/Contents/Developer/usr/bin/make  -C big-int
make[1]: *** /var: Is a directory.  Stop.
make: *** [big-int.dir] Error 2

[tautschnig]

I guess we should warn/fail if $(origin SRC) does not evaluate to file.