testsuite ignores CC parameter supplied to configure or make

On a 64-bit Linux system with biarch support, I built a 32-bit libffi with:
```
/path/to/configure --build=i386-unknown-linux-gnu --host=i386-unknown-linux-gnu CC='gcc -m32'
```
`make` then builds a 32-bit libffi, but `make check` ignores the CC setting (whether passed to configure or make) and always builds with just "gcc", which attempts to build the test programs as 64-bit (and thus failing to use the 32-bit libffi).