configure: Add missing $val != "yes" test to override compile_prog() result

configure: Add missing $val != "yes" test to override compile_prog() result

Since it's possible for some platforms to use statically configured
configurations (e.g. see around L350 for Cygwin), it should test if
the variable hasn't been set to "yes" before initializing the variable
with "no", so that compile_prog() result can be safely ignored when
the platform knows it's supported.

ca205a75 (configure: Make Cygwin take regular configure path) started
this due to a major change in the configuration path, which changed
Cygwin to take the normal path taken by all other platforms, and
these two were added after that change.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>