configure: use pkgconfig to detect samba

Recent Samba versions require pkg-config. We try that first but fall
back to old behaviour if smbclient.pc is missing.

This solves also an 'issue' with samba4 that correctly detects the
libsmbclient in the link test in configure, but fails to compile later
due to missing headers.

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Merge pull request #3537 from wsnipex/samba4

configure: use pkgconfig to detect samba