[pdal] PDAL 2.0.1 build issue

[Jim Klassen]

I am having build failures with PDAL on various Ubuntu 16.04 and Debian 9 machines that seems to have started sometime between 1.9 and 2.0.1.  It is looking for '/src/libgtest.so' (note the leading slash) which doesn't exist (there is no /src directory on my systems).  PDAL builds fine if I disable building tests.  I suspect something in cmake land is not right, but I am not sure where to look.  The '/src/libgtest.so' appears in many of the generated makefiles, but I don't see it set in CMakeCache.txt or see where it is calculated.

[ 41%] Linking CXX shared library /src/libgtest.so
/usr/bin/ld: cannot open output file /src/libgtest.so: No such file or directory
collect2: error: ld returned 1 exit status
vendor/gtest/CMakeFiles/gtest.dir/build.make:94: recipe for target '/src/libgtest.so' failed
make[2]: *** [/src/libgtest.so] Error 1
CMakeFiles/Makefile2:1320: recipe for target 'vendor/gtest/CMakeFiles/gtest.dir/all' failed
make[1]: *** [vendor/gtest/CMakeFiles/gtest.dir/all] Error 2
Makefile:160: recipe for target 'all' failed
make: *** [all] Error 2