Assorted fixes for BSD

Using BSD make and sh instead of GNU make and bash causes a several
problems which this patch should resolve.  Changes are:

* Change ${!...} bashism to $(eval echo \$...)
* Remove leading '-' from Makefile includes
* Use cat instead of <

Also, clock_gettime uses a different constant for CPU time, so modify
benchmark/benchmark.c to use that instead.

Signed-off-by: Evan Nemerson <evan@coeus-group.com>
build: don't assume wget is intalled

Signed-off-by: Evan Nemerson <evan@coeus-group.com>
build: be better about guarding unit tests and documentation

Signed-off-by: Evan Nemerson <evan@coeus-group.com>