Add proper compile definition when XTL has been configured

Basically so that -DENABLE_XTL_COMPLEX=On at configuration step implies
-DXSIMD_ENABLE_XTL_COMPLEX=1 for the tests.

Fix #942

test/CMakeLists.txt

@@ -191,7 +191,8 @@ else()
 endif()
 
 if(ENABLE_XTL_COMPLEX)
-target_include_directories(test_xsimd PRIVATE ${xtl_INCLUDE_DIRS})
+  add_compile_definitions(XSIMD_ENABLE_XTL_COMPLEX=1)
+  target_include_directories(test_xsimd PRIVATE ${xtl_INCLUDE_DIRS})
 endif()
 add_test(NAME test_xsimd COMMAND test_xsimd)