Disable installation step when Catch is used as a subproject

This is because otherwise the installations paths provided via
GNUInstallDirs become messed up and parts of the installation
package will end up in the wrong place.

Also it doesn't make much sense to force dependees to also install
our header alongside them.

Closes #1373

CMakeLists.txt

@@ -91,9 +91,12 @@ target_include_directories(Catch2
 # provide a namespaced alias for clients to 'link' against if catch is included as a sub-project
 add_library(Catch2::Catch2 ALIAS Catch2)
 
+# Only perform the installation steps when Catch is not being used as
+# a subproject via `add_subdirectory`, or the destinations will break,
+# see https://github.com/catchorg/Catch2/issues/1373
+if (NOT_SUBPROJECT)
     set(CATCH_CMAKE_CONFIG_DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/Catch2")
 
-include(CMakePackageConfigHelpers)
     configure_package_config_file(
         ${CMAKE_CURRENT_LIST_DIR}/CMake/Catch2Config.cmake.in
         ${CMAKE_CURRENT_BINARY_DIR}/Catch2Config.cmake
@@ -190,3 +193,5 @@ install(
       DESTINATION
         ${PKGCONFIG_INSTALL_DIR}
     )
+
+endif(NOT_SUBPROJECT)