Restructure the way libpam-sys-impls works to fix cross-compilation. The previous structure of libpam-sys-impls meant that things got confusing (including for me) between what constants were build-time and what constants were run-time. This broke cross-compilation. This simplifies the way that works so that `libpam-sys-impls` has *no* build script itself and is intended mostly as a library to be included in other libraries' build scripts (while also exporting the PamImpl enum).

# PAM configuration file for nonstick_testharness
#Sun#auth required pam_authtok_get.so
auth required pam_testharness.so param param2
account required pam_testharness.so
#Sun#password required pam_authtok_get.so
password required pam_testharness.so
session required pam_testharness.so