In Python 2.6 the process destructor emits a ResourceWarning if the
process has not terminated when destruction occurs. Fix by calling the
wait method before return.

The Popen class may be used as a context manager in Python 3.2+,
unfortunately that does not include 2.7.

The PIPE in the tests caused a ResourceWarning during testing in
python 3.

Closes gh-7707.  Happened for `pip install .` or another location (including a
remote git repo).

If there is no compiler on Linux, the error we were getting was CompileError
rather than the OSError the test was expecting.

This had the nasty side-effect of leaving us in a deleted temporary directory,
causing later test failures.

Try a check to see if we have a compiler and skip otherwise.

This replaces code of the pattern

```
fd, name = tempfile.mkstemp(...)
os.close(fd)
try:
   do stuff with name
finally:
    os.remove(name)
```

with

```
with temppath() as name:
    do stuff with name
```

A few more complicated cases are also handled. The remains some
particularly gnarly code the could probably be refactored to use
temppath, but that is a more demanding project.