How to use all *.c files in a directory with the Cmake build system?

How about the good old globbing?

FILE(GLOB MyCSources *.c)
ADD_EXECUTABLE(MyExecutable ${MyCSources})

Try this:

AUX_SOURCE_DIRECTORY

Find all source files in a directory.

AUX_SOURCE_DIRECTORY(dir VARIABLE) 

Collects the names of all the source files in the specified directory and stores the list in the variable provided. This command is intended to be used by projects that use explicit template instantiation. Template instantiation files can be stored in a "Templates" subdirectory and collected automatically using this command to avoid manually listing all instantiations.

It is tempting to use this command to avoid writing the list of source files for a library or executable target. While this seems to work, there is no way for CMake to generate a build system that knows when a new source file has been added. Normally the generated build system knows when it needs to rerun CMake because the CMakeLists.txt file is modified to add a new source. When the source is just added to the directory without modifying this file, one would have to manually rerun CMake to generate a build system incorporating the new file.