spkg-[install|check] should verify that SAGE_LOCAL is defined

Imported from <a href="http://trac.sagemath.org/sage_trac/ticket/633">trac #633</a>.

When building spkgs manually by invoking spkg-install on the shell I oftern do not have sage-env sourced. Consequently odd things happen:
```
[mabshoff@m940 mpfr-2.2.1.p1]$ ./spkg-install
./spkg-install: line 16: [: =: unary operator expected
```
We ought to verify that at least SAGE_LOCAL is non-empty. If it is empty print a warning message and exit.

To fix this we need to touch every spkg-install in the tree which is more that 75 scripts. So while we are at it we should also add spkg-check scripts to each spkg while we are at it.

Cheers,

Michael

Reported by: mabshoff

Component: packages

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