Add CMake option to disable upstream's catch download, we always want to use the 
system wide installed version. Gentoo bug 636828.

diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 86ce5a4..b36df56 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -10,7 +10,9 @@ list(APPEND CATCH_CMAKE_ARGS
     "-DNO_SELFTEST=true"
 )
 
-if(GIT_FOUND)
+option(FORCE_SYSTEM_CATCH "Force usage of system-wide catch install" OFF)
+
+if(NOT ${FORCE_SYSTEM_CATCH} AND GIT_FOUND)
     # add catch
     ExternalProject_Add(
         catch
-- 
2.13.6