[Bug-gnulib] addition: strcase.h

[Bruno Haible]

Ideally every function defined in a .c file should be declared in a .h
file. (To reduce surprises when you upgrade a file from gnulib and a
function's signature has changed, but your declaration in system.h has
not changed...)

Here's a beginning: I'll add strcase.h which declares strcasecmp() and
strncasecmp().

In order to use it, you remove the corresponding declarations from
your system.h and instead #include "strcase.h".

OK to commit?

Bruno