don't build tests by default

Index: unittest/CMakeLists.txt
--- unittest/CMakeLists.txt.orig
+++ unittest/CMakeLists.txt
@@ -47,7 +47,7 @@ endif()
 file(GLOB headers *.hpp)
 list(APPEND source_files ${headers})
 
-add_executable(unittest ${source_files})
+add_executable(unittest EXCLUDE_FROM_ALL ${source_files})
 
 if(MSVC AND NOT CMAKE_CXX_COMPILER_ID MATCHES "^Clang$")
   # Turn off /Zc:preprocessor for this test because it triggers a bug in some older Windows 10 SDK headers.