There are mainly two reasons why a source file is enforced by CMake:

1. To determine the LINKER_LANGUAGE from the file ending(s)
2. Not all compilers do support an object/library only link step (for details see below)

And if you move the main() function to library please keep the following in mind: Why does the order in which libraries are linked sometimes cause errors in GCC?

So if you build the libraries with CMake in the same project, I would recommend to change your libraries (at least the one containing your main() function) to an object library:

cmake_minimum_required(VERSION 2.8.8)

project(NoSourceForExe)

file(WRITE main.cc "int main() { return 0; }")

add_library(MyLibrary OBJECT main.cc)
add_executable(MyExecutable $<TARGET_OBJECTS:MyLibrary>)

The add_library() documentation lists a warning here:

Some native build systems may not like targets that have only object files, so consider adding at least one real source file to any target that references $<TARGET_OBJECTS:objlib>.

But those are rare and listed in Tests/ObjectLibrary/CMakeLists.txt:

# VS 6 and 7 generators do not add objects as sources so we need a
# dummy object to convince the IDE to build the targets below.
...
# Xcode does not seem to support targets without sources.

Not knowing which host OS(s) you are targeting, you may just give it a try.

References

• CMake Object Lib containing main
• CMake/Tutorials/Object Library

You forgot that you can now also alias imported targets that are flagged as global. :)

The error says what it means: there are no sources for libraries.

#adding entries for lib_third_party_ti_mmcsdlib
    include_directories("${PROJECT_SOURCE_DIR}/lib/third_party/ti/mmcsdlib")
    include_directories("${PROJECT_SOURCE_DIR}/lib/third_party/ti/mmcsdlib/include")
    add_library(lib_third_party_ti_mmcsdlib "") # NO SOURCES HERE!!!!
    target_compile_definitions(lib_third_party_ti_mmcsdlib 
        PRIVATE -DBOOT=MMCSD -DCONSOLE=UARTCONSOLE
    )
    subdirs("${PROJECT_SOURCE_DIR}/lib/third_party/ti/mmcsdlib")
    subdirs("${PROJECT_SOURCE_DIR}/lib/third_party/ti/mmcsdlib/include")

You should read the docs about add_library in cmake. If you don't provide any source files, you should declare it as INTERFACE

add_library(LibName INTERFACE)

In this case no compilation target would be generated. Otherwise, you can declare it as IMPORTED, then cmake will not try to create a target for compilation either.

For SHARED, STATIC or OBJECT you always need to supply sources.

You should check ProjectIncludes.cmake for what you really want to do: compile new libs or import them.

In CMAKE the add_executable and add_library are very similar. They just tell to CMAKE that it should create a MAKE instruction for a library or executable based on the list of src files that you provide after the name of the library/executable and options (such as SHARED, etc).

What you can do is to add the names of your libs that you want to link in a name variable that you increase such as

SET(TARGET_LIBS ${TARGET_LIBS} myFirstLib)
SET(TARGET_LIBS ${TARGET_LIBS} myNextLib)

and then simply:

target_link_libraries(myExe  ${TARGET_LIBS})

In this way you can easily define groups of libraries that may be needed for different sub projects without creating a meta-lib.

If all you want is a convenient target for use by others and don't care about whether you have one or multiple libraries, cmake can do that with an interface library:

add_library(library1 SHARED
    file1.cpp
    file2.cpp
)

add_library(library2 SHARED
    file3.cpp
    file4.cpp
)

add_library(master_library INTERFACE)

# Link the master library with the other libraries
target_link_libraries(master_library INTERFACE
    library1
    library2
)

Then in another location if you have

target_link_libraries(my_executable PRIVATE
    master_library
)

my_executable will link against library1 and library2