Build libiperf3 as both a shared library and a static library by default.

Use --disable-static or --disable-shared to build only one flavor
of libraries.

Tested on MacOS, FreeBSD, and CentOS 6 Linux.

Resolves #146.

Originally submitted by:	@i2aaron

(cherry picked from commit 3c4ef74e5eb2a33ee0eb54f7a884b2c8d45c2cf3)
Signed-off-by: Bruce A. Mah <bmah@es.net>
Regen to pick up changes from issue #146.