shell_surface: Add missing signal.h include
Improve package cflags/libs detection

- Provide a useful error message when the package could not be found.
- Declare all dependencies up front in the toplevel Makefile.
- Don't sort libraries; the order matters when statically linking.
Simplify silent rules
Use partial linking and objcopy --localize-hidden for libswc.a

This way, we don't expose internal symbols in the static library.
swc.pc: Properly set Requires{,.private}
drm: Drop support for GEM names