This is a CMake Environment Variable. Its initial value is taken from the calling process environment.

On UNIX one can use the DESTDIR mechanism in order to relocate the whole installation. DESTDIR means DESTination DIRectory. It is commonly used by makefile users in order to install software at non-default location. It is usually invoked like this:

make DESTDIR=/home/john install

which will install the concerned software using the installation prefix, e.g. /usr/local prepended with the DESTDIR value which finally gives /home/john/usr/local.

WARNING: DESTDIR may not be used on Windows because installation prefix usually contains a drive letter like in C:/Program Files which cannot be prepended with some other prefix.