I have a Raspberry Pico GPS library project. It comes with several examples. And each example can communicate to the GPS over I2C or UART.

Initially, it was a single-example project. I used a definition in the NMake config, that allowed you to build the project for either I2C or UART. Based on that parameter, it would generate a RP2040 .uf2 firmware that matched your choice.
I wasn't too happy with that approach, because a user was required to change the build file. And I had to run several builds when I was about to generate the assets for a new release.

Multiple Firmwares

If you analyse my challenge, it comes down to this:

• I have multiple examples that use my Teseo communication code
• The Teseo communication code can work on I2C or UART

I split the work up. First, I learned NMake to build two libraries for my communication code. One for I2C, and one for UART:

# sources shared by all examples with uart
add_library(${CMAKE_PROJECT_NAME}_shared_uart
        ${CMAKE_CURRENT_SOURCE_DIR}/gps_teseo_lib/teseo/teseo.cpp
        ${CMAKE_CURRENT_SOURCE_DIR}/port/pico/reset.cpp
        ${CMAKE_CURRENT_SOURCE_DIR}/port/pico/uart/teseo_communicate.cpp
)

target_include_directories(${CMAKE_PROJECT_NAME}_shared_uart PUBLIC 
        ${CMAKE_CURRENT_SOURCE_DIR}/gps_teseo_lib/teseo
        ${CMAKE_CURRENT_SOURCE_DIR}/gps_teseo_lib/callbackmanager
        ${CMAKE_CURRENT_SOURCE_DIR}/port/pico
        ${CMAKE_CURRENT_SOURCE_DIR}/port/pico/uart
)

target_link_libraries(${CMAKE_PROJECT_NAME}_shared_uart  PUBLIC
        pico_stdlib hardware_gpio hardware_uart
)

# sources shared by all examples with i2c
add_library(${CMAKE_PROJECT_NAME}_shared_i2c
        ${CMAKE_CURRENT_SOURCE_DIR}/gps_teseo_lib/teseo/teseo.cpp
        ${CMAKE_CURRENT_SOURCE_DIR}/port/pico/reset.cpp
        ${CMAKE_CURRENT_SOURCE_DIR}/port/pico/i2c/teseo_communicate.cpp
)

target_include_directories(${CMAKE_PROJECT_NAME}_shared_i2c PUBLIC 
        ${CMAKE_CURRENT_SOURCE_DIR}/gps_teseo_lib/teseo
        ${CMAKE_CURRENT_SOURCE_DIR}/gps_teseo_lib/callbackmanager
        ${CMAKE_CURRENT_SOURCE_DIR}/port/pico
        ${CMAKE_CURRENT_SOURCE_DIR}/port/pico/i2c
)

target_link_libraries(${CMAKE_PROJECT_NAME}_shared_i2c 
        pico_stdlib hardware_gpio hardware_i2c
)

Then, for each of my examples, I generate one binary that links to the I2C version. And one that links to the UART lib. Because the communication part is fully abstracted, I don't need conditional compilation. The linker can resolve it all.

# teseo reply response example --------------------------------------

add_executable(${CMAKE_PROJECT_NAME}_reply_response_uart
        ${CMAKE_CURRENT_SOURCE_DIR}/teseo_reply_response.cpp
)

target_link_libraries(${CMAKE_PROJECT_NAME}_reply_response_uart 
        ${CMAKE_PROJECT_NAME}_shared_uart
)

target_include_directories(${CMAKE_PROJECT_NAME}_reply_response_uart PUBLIC 
)

# select the debug output (not used by the GPS interface)
pico_enable_stdio_uart(${CMAKE_PROJECT_NAME}_reply_response_uart 1)
pico_enable_stdio_usb(${CMAKE_PROJECT_NAME}_reply_response_uart 0)

pico_add_extra_outputs(${CMAKE_PROJECT_NAME}_reply_response_uart )

add_executable(${CMAKE_PROJECT_NAME}_reply_response_i2c
        ${CMAKE_CURRENT_SOURCE_DIR}/teseo_reply_response.cpp
)

target_link_libraries(${CMAKE_PROJECT_NAME}_reply_response_i2c 
        ${CMAKE_PROJECT_NAME}_shared_i2c
)

target_include_directories(${CMAKE_PROJECT_NAME}_reply_response_i2c PUBLIC 
)

# select the debug output (not used by the GPS interface)
pico_enable_stdio_uart(${CMAKE_PROJECT_NAME}_reply_response_i2c 1)
pico_enable_stdio_usb(${CMAKE_PROJECT_NAME}_reply_response_i2c 0)

pico_add_extra_outputs(${CMAKE_PROJECT_NAME}_reply_response_i2c )

# nmea parse example --------------------------------------

add_executable(${CMAKE_PROJECT_NAME}_nmea_parse_uart
        ${CMAKE_CURRENT_SOURCE_DIR}/teseo_with_nmea_parse.cpp
)

target_link_libraries(${CMAKE_PROJECT_NAME}_nmea_parse_uart 
        ${CMAKE_PROJECT_NAME}_shared_uart
)

target_include_directories(${CMAKE_PROJECT_NAME}_nmea_parse_uart PUBLIC 
        ${CMAKE_CURRENT_SOURCE_DIR}/gps_nmea_lib/nmea
)

# select the debug output (not used by the GPS interface)
pico_enable_stdio_uart(${CMAKE_PROJECT_NAME}_nmea_parse_uart 1)
pico_enable_stdio_usb(${CMAKE_PROJECT_NAME}_nmea_parse_uart 0)

pico_add_extra_outputs(${CMAKE_PROJECT_NAME}_nmea_parse_uart )

add_executable(${CMAKE_PROJECT_NAME}_nmea_parse_i2c
        ${CMAKE_CURRENT_SOURCE_DIR}/teseo_with_nmea_parse.cpp
)

target_link_libraries(${CMAKE_PROJECT_NAME}_nmea_parse_i2c 
        ${CMAKE_PROJECT_NAME}_shared_i2c
)

target_include_directories(${CMAKE_PROJECT_NAME}_nmea_parse_i2c PUBLIC 
        ${CMAKE_CURRENT_SOURCE_DIR}/gps_nmea_lib/nmea
)

# select the debug output (not used by the GPS interface)
pico_enable_stdio_uart(${CMAKE_PROJECT_NAME}_nmea_parse_i2c 1)
pico_enable_stdio_usb(${CMAKE_PROJECT_NAME}_nmea_parse_i2c 0)

pico_add_extra_outputs(${CMAKE_PROJECT_NAME}_nmea_parse_i2c )

That's it. If you run NMake without target, it 'll build all permutations. If you only want a single firmware, you can define that as a target during build.
VSCode, and likely all of the other IDEs, have an option for that. For CLI users: you can also pass your (or no) target on the command line.

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