Link libc statically - cmake

Question

I am trying to make a static executable with CMake 3.15. I am building on Alpine Linux (hence with musl), and currently, my executable's ldd output is:
# ldd my_executable
/lib/ld-musl-x86_64.so.1 (0x7fc6f7977000)
libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x7fc6f65b3000)
libc.musl-x86_64.so.1 => /lib/ld-musl-x86_64.so.1 (0x7fc6f7977000)
libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0x7fc6f659f000)
I can set target_link_options(my_executable PRIVATE -static-libgcc -static-libstdc++), and they got linked statically:
# ldd my_executable
/lib/ld-musl-x86_64.so.1 (0x7fc6f7977000)
libc.musl-x86_64.so.1 => /lib/ld-musl-x86_64.so.1 (0x7fc6f7977000)
But I can't manage to get musl to link statically. I tried (with clean builds, to make sure that the configure step starts from zero):
Adding -static to target_link_options above.
set(CMAKE_EXE_LINKER_FLAGS "-static") before the call to add_executable(my_executable ...)
Adding -static to target_link_libraries(my_executable lib1 lib2 -static)
When I run CMake in VERBOSE=1 mode, it always ends the linking command with:
... -Wl,-Bdynamic -ldl -lrt -lm -lpthread
I believe that this is my issue: I want to get rid of that -Bdynamic. What am I missing? Is this -Bdynamic coming from one of my dependencies? I build them all from sources as static (.a) libraries, so how could they be linking libc dynamically? Or would I need to patch them all to add -static when I build them?

Answer 1

As hinted by KamilCuk's comment, the answer here seems to have the solution. Still, I'm not doing exactly the same, therefore I'll keep this answer, too.
For the target executable that I want statically linked:
add_executable(my_executable main.cpp)
I had to set the following properties/options:
set_target_properties(my_executable PROPERTIES LINK_SEARCH_START_STATIC ON)
set_target_properties(my_executable PROPERTIES LINK_SEARCH_END_STATIC ON)
target_link_options(my_executable PRIVATE -static-libgcc -static-libstdc++ -static)
Some notes:
LINK_SEARCH_*_STATIC were useful to remove -Bdynamic from the linking command.
I never managed to remove -ldl from the linking command, but it seems like dl did not get link eventually (presumably because it is not used).
ldd was not enough to verify that my_executable is statically linked. readelf -l my_executable showed that it does not have an INTERP header, and there is currently no such thing as a dynamic binary without it (using musl).
It turns out that checking whether a binary is statically linked or not is not so straightforward :-).