nanopb: build fails when used in external module

[sageve]

Describe the bug

The documentation describes how to use nanopb:

https://docs.zephyrproject.org/latest/services/serialization/nanopb.html

This works fine as long as it is used in the main CMakeLists.txt. If the same procedure is used in an external module the build fails with

CMake Error at C:/Users/KleiserS/Desktop/test/example/deps/modules/lib/nanopb/extra/FindNanopb.cmake:407 (target_compile_features):
  target_compile_features no known features for C compiler

  "GNU"

  version 12.2.0.

This happend in general before in #70516, was reverted and fixed later in #70658. I am not sure if it is fixed completely, because my module depending on nanopb fails now.

Commit 6b27cb2 is the last one working.

To Reproduce
I have prepared an example project. Steps to reproduce this behavior:

1. git clone https://github.com/sageve/nanopb-example
2. cd nanopb-example
3. west init -l
4. west update
5. west build -b qemu_cortex_a9 or any other board

Expected behavior
The build should not fail. In case I am misusing nanopb, it should be documented how to use it in modules.

[pdgendt]

The issue appears to be that the C standard isn't available yet when loading modules.

EDIT: A temporary fix can be to set the C11 in your module:

if (CONFIG_NANOPB_TEST)
    zephyr_library()

    set(CMAKE_C_COMPILE_FEATURES ${compile_features_c11})
    list(APPEND CMAKE_MODULE_PATH ${ZEPHYR_BASE}/modules/nanopb)
    include(nanopb)

    zephyr_library_sources(src/test.c)
    zephyr_include_directories(include)
endif()

EDIT2: The linked PR has a proper fix

[pdgendt]

@sageve PTAL at the linked PR, it should fix your issue