build: validate that system capstone works before using it

build: validate that system capstone works before using it

Some versions of capstone have shipped a broken pkg-config file which
puts the -I path without the trailing '/capstone' suffix. This breaks
the ability to "#include <capstone.h>". Upstream and most distros have
fixed this, but a few stragglers remain, notably FreeBSD.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210625172211.451010-2-berrange@redhat.com>
Message-Id: <20210709143005.1554-7-alex.bennee@linaro.org>