configure.ac: Don't hardcode -Wall and -Werror for configure checks, this fails with non-GCC compilers. Fixes bug #51...

Original commit message from CVS:
Patch by: Brian Cameron <brian dot cameron at sun dot com>
* configure.ac:
Don't hardcode -Wall and -Werror for configure checks, this fails
with non-GCC compilers. Fixes bug #517991.