Makefile: only set some BASIC_CFLAGS when RUNTIME_PREFIX is set

Makefile: only set some BASIC_CFLAGS when RUNTIME_PREFIX is set

Several build variables only have any meaning when the RUNTIME_PREFIX
variable has been set. In particular, the following build variables are
otherwise ignored:

    HAVE_BSD_KERN_PROC_SYSCTL
    PROCFS_EXECUTABLE_PATH
    HAVE_NS_GET_EXECUTABLE_PATH
    HAVE_ZOS_GET_EXECUTABLE_PATH
    HAVE_WPGMPTR

Make setting BASIC_CFLAGS, for each of these variables, conditional on
the RUNTIME_PREFIX being defined.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>