libcap: clean up configuration variables

libcap: clean up configuration variables

There's no need to set PACKAGECONFIG:class-native as we disable PAM in
the native DISTRO_FEATURES.

No need to set COPTS, since "Canonicalize build system"[1] the exported
CFLAGS/CPPFLAGS/LDFLAGS are respected.

Merge multiple make arguments into EXTRA_OEMAKE to remove duplication.

SYSTEM_HEADERS is not used upstream, remove.

[1] 2762c2c1a8c98d9012fcd40f20d133493a0b3219

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>