php: allow cache for strcasestr test

Fileinfo does an AC_TRY_RUN to test for strcasestr without allowing a
canned response, hence breaking cross-compiles. Use AC_CACHE_VAL. Fixes:
http://autobuild.buildroot.net/results/10a/10ae44cbc7c0c8e6f1ac54dae4c9d94c703aa3dd/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>