CID:1214163 Unchecked return value, check return value of switch_ivr_play_file and bail out if it fails

Putting `net-snmp-config --cflags` into CFLAGS causes major pollution;
it overrides optimization and debugging levels, warnings, and more.
While normally we do want to automatically locate library headers,
there has to be a better way to do this.  libsnmp is normally in the
usual place and doesn't need special handling.  Perhaps people with
libsnmp in a weird place should just need to add the -I flag to their
CFLAGS before build.

Calling out to net-snmp-config --agent-libs causes transitive
dependencies to get pulled in, but we don't need those -- a sensible
dynamic linker pulls those in automatically.  Trying to track the
transitive dependencies manually would be a losing battle.

People were recently hitting this on Debian sid/jessie, where libpci
is in the transitive dependency list but isn't otherwise one of our
build dependencies.

FS-6517 skypopen: mod_skypopen not using configured context with mod_sms chatplan with PATCH, thanks Russell Treleaven

This allows someone without pkg-config in their PATH to set where
pkg-config is located.

We depend on pkg-config, but we weren't explicitly testing for it, so
the absence of pkg-config would cause the sqlite test to fail, which
was confusing.

This makes portable the fix from commit
28ee7736.