-D <var>:<type>=<value>, -D <var>=<value>
Create or update a cmake cache entry.
When cmake is first run in an empty build tree, it creates a CMakeCache.txt file and populates it with customizable settings for the project. This option may be used to specify a setting that takes priority over the project’s
default value. The option may be repeated for as many cache entries as desired.
If the :<type> portion is given it must be one of the types specified by the set() command documentation for its CACHE signature. If the :<type> portion is omitted the entry will be created with no type if it does not exist with a
type already. If a command in the project sets the type to PATH or FILEPATH then the <value> will be converted to an absolute path.
This option may also be given as a single argument: -D<var>:<type>=<value> or -D<var>=<value>.
cmake -DCOMPILE_OPENCL12 ../
doesn't work
maybe it should be
cmake -DCOMPILE_OPENCL12=On ..
instead?
From cmake man page: