[CMake] Set a variable?

[motes motes]

In a CMakeList.txt file I have:

SET(ELASTIX_BUILD E:/local/lib/elastix_sources_v4.1/src/build)
...
LINK_DIRECTORIES(${ELASTIX_BUILD}/Components/Optimizers/StandardGradientDescent)




I would to be able to specify the variable: ELASTIX_BUILD when I read
the CMakeList.txt file and then choose it from the file browser. I
have tried:

FIND_PACKAGE(ELASTIX_BUILD REQUIRED)

Its now possible to select the path when I run CMake, but I get an
error when I press configure:

....which is not a directory containing a package configuraion file....

Any ideas on how to specify that a variable should be set when running CMake?