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).

/*
 * Semi-public functions exposed in Sun's `libpam.so`.
 * https://code.illumos.org/plugins/gitiles/illumos-gate/+/f3a1073761f959966ab629695ee17f4417413796/usr/src/lib/libpam/pam_impl.h
 */

#define PAM_PROMPT 1
#define PAM_HANDLE 2

extern int __pam_get_authtok(
    pam_handle_t *pamh,
    int source,
    int type,
    char *prompt,
    char **authtok
);

extern void __pam_log(
    int priority,
    const char *format,
    ...
);