waf: link tests with gtest library from submodule

That makes it possible to build tests on distributions that don't provide
official packages for gtest. For example, the travis environment is Ubuntu,
which doesn't have official package for gtest library.

wscript

@@ -80,6 +80,7 @@ def configure(cfg):
     cfg.load('waf_unit_test')
     cfg.load('mavgen')
     cfg.load('gbenchmark')
+    cfg.load('gtest')
     cfg.load('static_linking')
 
     cfg.start_msg('Benchmarks')
@@ -88,12 +89,11 @@ def configure(cfg):
     else:
         cfg.end_msg('disabled', color='YELLOW')
 
-    cfg.env.HAS_GTEST = cfg.check_cxx(
-        lib='gtest',
-        mandatory=False,
-        uselib_store='GTEST',
-        errmsg='not found, unit tests disabled',
-    )
+    cfg.start_msg('Unit tests')
+    if cfg.env.HAS_GTEST:
+        cfg.end_msg('enabled')
+    else:
+        cfg.end_msg('disabled', color='YELLOW')
 
     cfg.env.prepend_value('INCLUDES', [
         cfg.srcnode.abspath() + '/libraries/',
@@ -117,6 +117,7 @@ def list_boards(ctx):
     print(*boards.get_boards_names())
 
 def build(bld):
+    bld.load('gtest')
 
     #generate mavlink headers
     bld(