• Make the port safe to build as root. [1]

Given an absolute /path Linux programs first try to access
/compat/linux/path before trying /path. That means mkdir -p ${STAGEDIR}
will always create /compat/linux${STAGEDIR} when run as root and echo >
/dev/null will create /compat/linux/dev/null as a regular file. Fix this
by running configure with FreeBSD /bin/sh and build/install using FreeBSD
gmake. PATH is set to /compat/linux/usr/bin and MKDIR_P to /bin/mkdir -p
so all commands except mkdir are still Linux programs.

• Add USES=libtool:build because libtoolize is needed. [2]
• Add USES=pkgconfig.
• Remove -DFREEBSD_OSS_BUFSZ_P2 because it breaks some games. [3]
• Replace -nostdinc -isystem ... with --sysroot=${LINUXBASE}. [3]
• Include bsd.port.options.mk to get ARCH. [3]

PR: 240043
Reported by: VVD <vvd@unislabs.com> [1]
Reported by: Giacomo Olgeni <olgeni@olgeni.com> [2]
Reported by: Alex S <iwtcex@gmail.com> [3]