[CMake] Replacing compiler flags for certain project subdirectories

[Whitcomb, Mr. Tim]

I'm in the process of adding Cmake-build capability to a Fortran project that currently follows a traditional recursive-make build structure.  I've converted all the makefiles to CMakeLists.txt files and can now produce all the target libraries and executables with their dependencies correctly recognized.  These dependencies have made a huge difference in allowing us to perform parallel builds and have the project ready to go *much* faster than before.  I have a top-level CMakeLists.txt file that contains a few library searches (e.g. LAPACK, BLAS) and then an add_subdirectory call for each subdirectory in the project.  Each subdirectory corresponds to a library and/or an executable.

Now that the listing files contain all the sources files they need, my current task is to go through and set the proper compiler/preprocessor flags to match the original makefiles.  We have several directories in our source tree that are auxiliary libraries that we store in our Subversion repository and build as part of our regular build process.  The libraries that are built in these directories are compiled with different compiler flags and different preprocessor definitions than the rest of the project.  I've been able to handle the preprocessor definitions by using add_definitions in the CMakeLists.txt files in the lower-level directories.  I see based on reading through some past threads (including the recent "Different CMAKE_CXX_FLAGS for different executables") on this list and on some StackOverflow questions that the COMPILE_FLAGS target property is very close to what I need (and actually *was* what I needed in several cases) but only appends flags to those currently in use.  What I need is to be able to define a new set of compile flags (i.e. "forget everything you were using before and use THIS set in this directory").

I've started going through the documentation for Building External Projects[*] but the first line states that "[a]n "external project" is one that you can get the source code for, but does not readily build with a simple ADD_SUBDIRECTORY call in your CMakeLists.txt file."  This feels like it's designed to solve a slightly different problem than what I'm trying to do - I have an add_subdirectory call in my CMakeLists.txt file - is the proper fix for this issue to make the pieces that require different flags external projects?  Will making some of these libraries external projects mess with the dependency calculation?

I'm just getting started and trying to wrap my head around the options available.  What is the standard way of dealing with this issue?  Is there anything I'm missing?  Thank you for your assistance!

Tim
[w] 

[*] http://www.kitware.com/products/html/BuildingExternalProjectsWithCMake2.8.html