configure: fix run-compiler for old /bin/sh

configure: fix run-compiler for old /bin/sh

If you try to assign and export on the same line on some older /bin/sh
implementations, it complains:

```
$ export "NAME=value"
NAME=value: is not an identifier
```

This commit rewrites run-compiler's assignments and exports to work with
old /bin/sh, splitting assignment and export into two separate
statements, and only quote the value. So now we have:

```
NAME="value"
export NAME
```

While we're here, make the same change to the two supporting
assign+export lines preceeding the script to be consistent with how
exports work throughout the rest of configure.ac.

Closes #11228