Specify include directories as system includes

[Randalphwa]

Migrated from vector-of-bool/cmrc issue #41 by @

When using strict compiler warnings with clang, the header cmrc.hpp emits conversion warnings.
When building a project using cmrc, such a warning should not appear to the user, so I woud propose to specify the include directory as a system include to silence warnings:

target_include_directories(cmrc-base SYSTEM INTERFACE $<BUILD_INTERFACE:${CMRC_INCLUDE_DIR}>)