- Add LIBS="${LIBS}" to MAKE_ENV and CONFIGURE_ENV.
- Add an option helper for LIBS.
- Adjust all ports that already use LIBS.  Also remove references to
  PTHREAD_CFLAGS and PTHREAD_LIBS while here.
- Some ports did not support having a LIBS environment variable and
  required additional patches.

Somewhat simplified a linker command line looks like:

${CC} ${src_LDFLAGS} ${LDFLAGS} ${src_LIBS} ${LIBS}

where src_LDFLAGS and src_LIBS are controlled by upstream and LDFLAGS and
LIBS can be controlled by us.  If possible -L and -l flags need to be
added to LIBS to make sure they appear after any -L and -l flags set by
upstream.  Many ports currently add -L${LOCALBASE}/lib to LDFLAGS but this
may appear too early on the command line causing installed libraries to
be linked in instead of freshly built ones.

Additional changes:
benchmarks/netio: Replace WITH_IPV6 with an IPV6 option.
comms/gnokii: Replace some patches with USES=pathfix.  Also remove -fPIC.
graphics/gimageview: USES=libtool and install desktop file in DESKTOPDIR.
graphics/visionworkbench: Remove FreeBSD 7 support.
multimedia/libmovtar: New LIB_DEPENDS syntax.
multimedia/opencinematools: Use standard do-build.
net/siproxd: USES=libtool:keepla (port actually needs .la files for plugins)
net-mgmt/nagios: Remove -fPIC.
net-mgmt/nagios4: Remove -fPIC.
print/cups-base: Only add -lssp_nonshared on i386 and OSVERSION < 1000036.
security/p11-kit: Replace PTHREAD_LIBS in CONFIGURE_ENV with
ac_cv_func_pthread_mutexattr_init=no in CONFIGURE_ARGS.  This skips a test
in configure that falsely detects pthread_mutexattr_init in our libc.
sysutils/dar: Fix iconv detection.
x11/rxvt-unicode: Remove -lstdc++ and patch configure to remove a FreeBSD
hack and use $CXX as linker as on other platforms.

PR:		190592
Exp-run by:	antoine
Approved by:	portmgr (antoine)