Ubuntu build fails to find sys/audioio.h

[philburk]

The configure script for Ubuntu 18.04 fails because of a missing <sys/audioio.h>

To Reproduce

1. Check out latest code on Ubuntu 18.04
2. autoconf
3. ./configure
4. make

Expected behavior
Successful build.

Actual behavior

libtool: compile: gcc -c -g -O2 -DPA_LITTLE_ENDIAN -I./include -I./src/common -I./src/os/unix -pthread -DPACKAGE_NAME="" -DPACKAGE_TARNAME="" -DPACKAGE_VERSION="" -DPACKAGE_STRING="" -DPACKAGE_BUGREPORT="" -DPACKAGE_URL="" -DSTDC_HEADERS=1 -DHAVE_SYS_TYPES_H=1 -DHAVE_SYS_STAT_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 -DHAVE_MEMORY_H=1 -DHAVE_STRINGS_H=1 -DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1 -DHAVE_UNISTD_H=1 -DHAVE_DLFCN_H=1 -DLT_OBJDIR=".libs/" -DHAVE_SYS_SOUNDCARD_H=1 -DHAVE_LINUX_SOUNDCARD_H=1 -DSIZEOF_SHORT=2 -DSIZEOF_INT=4 -DSIZEOF_LONG=8 -DHAVE_CLOCK_GETTIME=1 -DHAVE_NANOSLEEP=1 -DPA_USE_ALSA=1 -DPA_USE_AUDIOIO=1 -DPA_USE_JACK=1 -DPA_USE_OSS=1 src/hostapi/audioio/pa_unix_audioio.c -fPIC -DPIC -o src/hostapi/audioio/.libs/pa_unix_audioio.o
src/hostapi/audioio/pa_unix_audioio.c:46:10: fatal error: sys/audioio.h: No such file or directory
#include <sys/audioio.h>
^~~~~~~~~~~~~~~

Desktop (please complete the following information):

• OS: [e.g. Mac OS] Ubuntu
• OS Version [e.g. 22] 18.04
• PortAudio version: stable, nightly snapshot (which?), current (please give date and/or Git hash): master
• If Windows or Linux, which Host API (e.g. WASAPI): default

Additional context
Caused by #645

./configure prints:

checking sys/audioio.h usability... no
checking sys/audioio.h presence... no
checking for sys/audioio.h... no

AudioIO ..................... no