Allow build to proceed if gtest is missing

[rkday]

Fixes #637 and #626 .

If gtest is missing, you can now build the sipp binary. sipp_unittest still depends on gtest, and attempts to build that with it missing will fail with:

/home/rkd/src/sipp/src/sipp_unittest.cpp:22:10: fatal error: gtest/gtest.h: No such file or directory
   22 | #include "gtest/gtest.h"
      |          ^~~~~~~~~~~~~~~
compilation terminated.