Add define that pulls in reporter and listeners interfaces

This allows users to define reporters and listeners in files different
from the main file.

Related to #991, #986
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c98c326..b538f5d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -138,6 +138,7 @@
         ${HEADER_DIR}/internal/catch_enforce.h
         ${HEADER_DIR}/internal/catch_errno_guard.h
         ${HEADER_DIR}/internal/catch_exception_translator_registry.h
+        ${HEADER_DIR}/internal/catch_external_interfaces.h
         ${HEADER_DIR}/internal/catch_fatal_condition.h
         ${HEADER_DIR}/internal/catch_impl.hpp
         ${HEADER_DIR}/internal/catch_interfaces_capture.h