安装mpd5出现报错"/usr/share/mk/bsd.kmod.mk", line 12: "can't find kernel source tree"

Note: If there is not a /usr/src/sys directory on your system, then the kernel source has not been installed. The easiest way to do this is by running sysinstall as root, choosing Configure, then Distributions, then src, then base and sys. If you have an aversion to sysinstall and you have access to an “official” FreeBSD CDROM, then you can also install the source from the command line:

????# mount /cdrom
????# mkdir -p /usr/src/sys
????# ln -s /usr/src/sys /sys
????# cat /cdrom/src/ssys.[a-d]* | tar -xzvf -
????# cat /cdrom/src/sbase.[a-d]* | tar -xzvf -

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cd
?make
?make install