strings/memory: constify and func_attr.

Fix MB_COPY_cHAR() to accept const pointers.
strings: Enable -Wconvert.
Merge pull request #1497 from splinterofchaos/const-attr

constify and func-attribute memory.c and strings.c
Wconversion: fix #1578
Travis: Perform installation test on 32-bit build.
Travis: Add flag to force building of dependencies.
Test that setting the deps flag works.