Compatibility with strict(er) C99 compilers

[Florian Weimer]

hplip uses implicit ints and implicit function declarations in a few places. This causes the build to fail with a compiler that does not implement these obsolete C language features (they were removed with C99).

With the attached patches, I can get the sources to build again with a more strict compiler.

I added the manual strcasestr declaration only to get the build going. I've proposed that we expose the declaration by default in future glibc versions. I did not want to switch the entire build to -D_GNU_SOURCE out of compatibility concerns, but this is what you should do if you want to use GNU extensions such as strcasestr.