CMake, multiple targets (asan tsan..) without having to recompile everything - cmake

Question

Goal
I want to define several targets:
make msan: compiles the code with clang with memory sanitizer
make tsan: compiles the code with clang with thread sanitizer
make : compiles the code with gcc
And be able to easily switch between them.
For example I don't want each time I switch rebuild all my objects, (I will have to do it the first time of course, but later if I modify a file and I do make and then make asan it should recompile only this file for each target)
What I have done so far
I have managed to create these targets and from the root directory, but each time I have to do a make clean and recompile.
option(CLANG_MSAN "Enable Clang memory sanitizer" OFF)
if (CLANG_MSAN)
set (CMAKE_CXX_FLAGS "-g -fsanitize=address -fno-omit-frame-pointer")
endif()
add_custom_target(asan
COMMAND ${CMAKE_COMMAND}
-DCLANG_MSAN=ON
-DCMAKE_CXX_COMPILER=clang++
-DCMAKE_C_COMPILER=clang)
Is it possible to do such a thing with CMake?

Answer 1

Yes, but use multiple build directories:
Create a build directory per configuration.
Configure your project in your build directories with the parameters you need. E.g. cmake -DCMAKE_COMPILER=clang -DCMAKE_C_FLAGS="-fsanitize=thread" .. or the stuff from your question.
If you switch the build directory, you changed your setup.
This implies out-of-source builds, which are encouraged by CMake anyway.

Answer 2

I believe you should be able to achieve this with the ExternalProject module. You could add three external projects, one for msan, one for tsan, and one for the basic GCC build. The two sanitiser builds would be marked as EXCLUDE_FROM_ALL 1.
The CMakeLists for all three of them could share the common part via include().
Something like this:
Root CMakeLists.txt
ExternalProject_Add(msan
EXCLUDE_FROM_ALL 1
SOURCE_DIR msan
CMAKE_GENERATOR ...
)
ExternalProject_Add(tsan
EXCLUDE_FROM_ALL 1
SOURCE_DIR tsan
CMAKE_GENERATOR ...
)
ExternalProject_Add(normal
SOURCE_DIR src
CMAKE_GENERATOR ...
)
src/CMakeLists.txt
include(common.cmake)
src/common.cmake
# Normal CMake code for your project
add_library(...)
msan/CMakeLists.txt
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g -fsanitize=address -fno-omit-frame-pointer")
include(../src/common.cmake)