do not pass bogus $(SRC_PATH) include paths to cc during configure

Non-existent -I paths are dropped silently by the compiler, but still
it is not polite to pass bogus options.  Configure-time tests do not
need any include files from the source path, so only include -I flags
at make time (when they're properly expanded).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>