gtest: force static library build

While it seems possible to build gtest as a shared library, using it
in other projects requires to set some specific configuration flags
in these projects for correctly linking against the shared version
of the gtest library.

So, to avoid bothering many projects and keep things as they were so
far, we just force gtest to be built as a static library.

Fixes:
  http://autobuild.buildroot.net/results/20cbf90749672607cb277c676a45d2c0c3965fa0/
  http://autobuild.buildroot.net/results/5c6e58c51b2a14a7f23d296f771a76096b80362a/
  http://autobuild.buildroot.net/results/bde777d343e9c4884272893d0da3b113dcc75e3a/
  http://autobuild.buildroot.net/results/20cbf90749672607cb277c676a45d2c0c3965fa0/
  http://autobuild.buildroot.net/results/89a29d1f597dbc12a6fc8eb079133a84952fc612/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>