Is it possible to get CMake to build both a static and shared version of the same library?

Yes, it's moderately easy. Just use two "add_library" commands:

add_library(MyLib SHARED source1.c source2.c)
add_library(MyLibStatic STATIC source1.c source2.c)

Even if you have many source files, you would place the list of sources in a cmake variable, so it's still easy to do.

On Windows you should probably give each library a different name, since there is a ".lib" file for both shared and static. But on Linux and Mac you can even give both libraries the same name (e.g. libMyLib.a and libMyLib.so):

set_target_properties(MyLibStatic PROPERTIES OUTPUT_NAME MyLib)

But I don't recommend giving both the static and dynamic versions of the library the same name. I prefer to use different names because that makes it easier to choose static vs. dynamic linkage on the compile line for tools that link to the library. Usually I choose names like libMyLib.so (shared) and libMyLib_static.a (static). (Those would be the names on linux.)

Since CMake version 2.8.8, you can use "object libraries" to avoid the duplicated compilation of the object files. Using Christopher Bruns' example of a library with two source files:

# list of source files
set(libsrc source1.c source2.c)

# this is the "object library" target: compiles the sources only once
add_library(objlib OBJECT ${libsrc})

# shared libraries need PIC
set_property(TARGET objlib PROPERTY POSITION_INDEPENDENT_CODE 1)

# shared and static libraries built from the same object files
add_library(MyLib_shared SHARED $<TARGET_OBJECTS:objlib>)
add_library(MyLib_static STATIC $<TARGET_OBJECTS:objlib>)

From the CMake docs:

An object library compiles source files but does not archive or link their object files into a library. Instead other targets created by add_library() or add_executable() may reference the objects using an expression of the form $<TARGET_OBJECTS:objlib> as a source, where objlib is the object library name.

Simply put, the add_library(objlib OBJECT ${libsrc}) command instructs CMake to compile the source files to *.o object files. This collection of *.o files is then referred to as $<TARGET_OBJECT:objlib> in the two add_library(...) commands that invoke the appropriate library creation commands that build the shared and static libraries from the same set of object files. If you have lots of source files, then compiling the *.o files can take quite long; with object libraries you compile them only once.

The price you pay is that the object files must be built as position-independent code because shared libraries need this (static libs don't care). Note that position-independent code may be less efficient, so if you aim for maximal performance then you'd go for static libraries. Furthermore, it is easier to distribute statically linked executables.

There is generally no need to duplicate ADD_LIBRARY calls for your purpose. Just make use of

$> man cmake | grep -A6 '^ *BUILD_SHARED_LIBS$' 
   BUILD_SHARED_LIBS
          Global flag to cause add_library to create shared libraries if on.

          If present and true, this will cause all libraries to be built shared unless the library was
          explicitly added as a static library.  This variable is often added to projects as an OPTION
          so  that each user of a project can decide if they want to build the project using shared or
          static libraries.

while building, first (in one out-of-source directory) with -DBUILD_SHARED_LIBS:BOOL=ON, and with OFF in the other.