this checks whether -Werror=unknown-warning-option and
-Werror=unused-command-line-argument are supported by the toolchain. if so,
it adds those flags to the tryflag and tryldflag checks in ./configure.

so far only clang supports these flags, but clang is also the only toolchain
I've encountered that ignores unrecognized arguments instead of erroring out.
---
 configure | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)