Don't check and install bundled Boost headers if CXX_API not required

Author: Growl1234

export/CMakeLists.txt.export

@@ -796,7 +796,7 @@ install(CODE "
 ")
 
 # install bundled Boost headers if needed
-if (NOT LIBINT_HAS_SYSTEM_BOOST_PREPROCESSOR_VARIADICS)
+if (LIBINT2_REQUIRE_CXX_API AND NOT LIBINT_HAS_SYSTEM_BOOST_PREPROCESSOR_VARIADICS)
     install(
       DIRECTORY ${PROJECT_BINARY_DIR}/include/libint2/boost
       DESTINATION "${LIBINT2_INSTALL_INCLUDEDIR}/libint2"