CMake: Convert relative path to absolute path, with build directory as current directory

From the docs of get_filename_component (highlighting be me)
:

get_filename_component(<VAR> <FileName>
                       <COMP> [BASE_DIR <BASE_DIR>]
                       [CACHE])

Set <VAR> to the absolute path of <FileName>, where <COMP> is one of:

ABSOLUTE = Full path to file
REALPATH = Full path to existing file with symlinks resolved

If the provided <FileName> is a relative path, it is evaluated relative to the given base directory <BASE_DIR>. If no base directory is provided, the default base directory will be CMAKE_CURRENT_SOURCE_DIR.

Paths are returned with forward slashes and have no trailing slashes. If the optional CACHE argument is specified, the result variable is added to the cache.

Thus, you use:

get_filename_component(buildDirRelFilePath "${myFile}"
                       REALPATH BASE_DIR "${CMAKE_BINARY_DIR}")

To convert an absolute path to a file into a relative paths, you might use the file command:

file(RELATIVE_PATH <variable> <directory> <file>)

Compute the relative path from a <directory> to a <file> and store it in the <variable>.

file(RELATIVE_PATH buildDirRelFilePath "${CMAKE_BINARY_DIR}" "${myFile}")