EESSI-extend: Use flags as value for EASYBUILD_SEARCH_PATH_CPP_HEADERS (to rely on $CPPFLAGS in build environment instead of redefining $C_INCLUDE_PATH & co)

EESSI-extend: Use flags as value for EASYBUILD_SEARCH_PATH_CPP_HEADERS (to rely on $CPPFLAGS in build environment instead of redefining $C_INCLUDE_PATH & co) #815