https://bugs.gentoo.org/978866#c8

Thanks to  Bernd Feige.

--- a/ac/gtest.m4
+++ b/ac/gtest.m4
@@ -35,5 +35,23 @@ AC_DEFUN([AX_GTEST],[
   AC_SUBST(GTEST_TYPE)
 ])
 
-AX_GTEST
+AC_ARG_ENABLE(
+  [tests],
+  AS_HELP_STRING([--disable-tests],[Disable building of unit tests (default: auto)]),
+  [],
+  [enable_tests=auto]
+)
+
+if test x"$enable_tests" != xno; then
+  AX_GTEST
+else
+  GTEST_CFLAGS=
+  GTEST_LIBS=
+  GTEST_TYPE=no
+  AC_SUBST([GTEST_CFLAGS])
+  AC_SUBST([GTEST_LIBS])
+  AC_SUBST([GTEST_TYPE])
+  AC_MSG_NOTICE([Tests disabled; not looking for gtest])
+fi
+
 AC_PATH_PROG(VALGRIND, valgrind,, $PATH)