Makefile: link gtest statically

Makefile: link gtest statically

The problem:

 - the unittests link against gtest, and gtest is not installed.  that's
   normally fine, but...
 - rbd and rados api unit tests link against gtest, and are installed
   by 'make install'.  they are needed for teuthology runs, etc.
 - if we build gtest as an .la library, we can only control whether *all*
   or *no* .la libraries are linked statically.
 - we want librados to be linked dynamically.

The solution:

 - build gtest as .a instead of a libtool library
 - link it statically, always.

Unit test binaries are bigger now.  Oh well...

Fixes: #2331
Signed-off-by: Sage Weil <sage@inktank.com>