Porting test improvements from multi-language-branch:

Porting test improvements from multi-language-branch:
* tests/defs: Find and set the value of CC probed by libtool.m4
* tests/link-2.test: Use a .lo wrapper script, and the value of
CC set by defs instead of hardcoding gcc.
* tests/link.test: Use the value of CC set by defs instead of
hardcoding gcc.
* tests/quote.test: Ditto.
* tests/suffix.test: Ditto.