hints for CPU model Marvell Kirkwood mv6282 ARMv5te

Tested on Synology DS213

architecture arm5tel seems to differ from other ARM

On Synology, all development is installed under /opt
LANG settings other than C are not (really) supported
Search for errno.h in more (semi)default locations
With symlinks in /lib m and crypt are are no longer exceptions

Configure will only look for libfoo.so. If libfoo.so.{num} exists and
libfoo.so does not, make a symbolic link and start over. For this
Synology DS213, I needed

 # cd /lib
 # ln -s libm.so.6 libm.so
 # ln -s libcrypt.so.1 libcrypt.so
Enhance INSTALL instructions for missing libraries
Note (new) platform support

Do I need to add some more extensive reading in README.arm or the like?