samples: external_lib: Include --target in exported build flags

Appropriately setting the `--target` flag is necessary when using clang
and building for a target other than the default. Zephyr generally
accomplishes this by setting the CMAKE_<LANG>_COMPILER_TARGET variables and
allowing cmake to automatically provide the `--target` flag when building.

For the external_lib sample, however, cmake can't add the flag and it was
not otherwise exported. As such, clang typically threw errors when building
this sample for any non-default targets due to mismatches between
target-specific flags and the default target.

To fix this, ensure we select the correct target by checking if
CMAKE_C_COMPILER_TARGET has been defined and adding
`--target=<triple>` to the list of flags that are exported if so.

Signed-off-by: Jonathon Penix <jpenix@quicinc.com>

samples/application_development/external_lib/CMakeLists.txt

@@ -19,8 +19,12 @@ zephyr_get_system_include_directories_for_lang_as_string(C system_includes)
 zephyr_get_compile_definitions_for_lang_as_string(       C definitions)
 zephyr_get_compile_options_for_lang_as_string(           C options)
 
+if(DEFINED CMAKE_C_COMPILER_TARGET)
+  set(target_flag "--target=${CMAKE_C_COMPILER_TARGET}")
+endif()
+
 set(external_project_cflags
-  "${includes} ${definitions} ${options} ${system_includes}"
+  "${target_flag} ${includes} ${definitions} ${options} ${system_includes}"
   )
 
 include(ExternalProject)