shadow: don't compile bcrypt for unsupported platforms

shadow: don't compile bcrypt for unsupported platforms

If musl has no bcrypt:

passwd: failed to crypt password with salt '$2a$13$w8EJ0Yfz5bGsG4U/0m7bk/': Function not implemented
The password for root is unchanged.

glibc output as it has no bcrypt:

passwd: failed to crypt password with salt '$2a$13$xbpmAYmq6Q/rZN5jOlNxJZ': Invalid argument
The password for root is unchanged.

--without-bcrypt output:

Invalid ENCRYPT_METHOD value: 'BCRYPT'.
Defaulting to DES.
passwd: password changed.

The solution was tested on glibc despite using a musl specific variable
Still works.

Signed-off-by: Rosen Penev <redacted>