CMake: Conditional build fixes and updates.

HAVE_PORTAUDIO_H didn't work for Stig. Use PORTAUDIO_FOUND instead,
which is the correct name to use according to the
FIND_PACKAGE_HANDLE_STANDARD_ARGS documentation. Use xxx_FOUND in a
couple of other places.

Conditionally build caputils/airpcap_loader.c.

Change-Id: I6a134192902229a446dccf43bfb7bbfe2a55d1e2
Reviewed-on: https://code.wireshark.org/review/4291
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Stig Bjørlykke <stig@bjorlykke.org>