build: Use flags returned by pkg-config to find smbclient

Previously, the result returned by PKG_CHECK_MODULES is only used to
find header files, and --with-samba-libs option is required if the
shared library is not installed in the default search path. This commit
replaces the old detection code with simple pkg-config calls, making
it easier to find and specify the smbclient library.

https://bugzilla.gnome.org/show_bug.cgi?id=782446