meson: fix str[n]casecmp detection on clang-cl

Meson incorrectly detects strcasecmp, strncasecmp on clang-cl if 'prefix:'
is not specified for cc.has_function().


Fixes GNOME/glib#2337

Before this change:
msvc was using _stricmp()
gcc on mingw was using strcasecmp()
gcc on linux was using strcasecmp()
clang-cl was trying to use strcasecmp()

After this change:
msvc is using _stricmp()
gcc on mingw is using strcasecmp()
gcc on linux is using strcasecmp()
clang-cl is using _stricmp()

Tests are still failing to build with clang-cl, but that's a separate issue.
10 jobs for fix-clang-cl-strcasecmp in 12 minutes and 56 seconds (queued for 1 second)