Use FindALSA provided by cmake

Summary:
...instead of using a copy from KDELibs4Supprt. It has been included
since cmake-2.8.0.

Furthermore drop the special casing for <sys/asoundlib.h> which has
been deprecated since 2001, according to [1].

[1] http://git.alsa-project.org/?p=alsa-lib.git;a=commitdiff;h=6157018e8ad7179271791f960570db79b23a481d

Differential Revision: https://phabricator.kde.org/D3862