contrib: use the built-in CMake POSITION_INDEPENDENT_CODE system

It may use a different flag depending on the target and disable it on platforms not supporting it.

So CMake projects already set the value internally when they need it, others we forced the PIC flag value manually.

Of all the 31 CMake targets, only these don't use PIC:

• fluid
• freetype
• ogg
• projectM
• zlib

Only gsm and lua are left using HOSTVARS_PIC. Notably 2 projects using a custom/manual Makefile with no configure script.