adding a #define through Cmake not working at compilation

Question

I have a certain file, a.hpp, of the following structure:

#ifndef HEADER
#define HEADER

#ifdef FLAG
namespace Foo {
   void somefunc();
}
#endif // FLAG

#endif // HEADER

I would like to include the file in another file, b.hpp, and not provide the #define FLAG in that file, but rather provide it using the Cmake file.

the a.hpp file is a part of an external library called Utils, and is added via an ExternalProject_Add command like this:

target_link_libraries(Proj Proj/Lib/LibUtils.a)
ExternalProject_Add(Utils
   GIT_REPOSITORY http://somerepo
   GIT_TAG master
   CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=Proj/Lib -DFLAG
)
ExternalProject_Get_Property (Utils SOURCE_DIR)
target_include_directories(Proj PRIVATE ${SOURCE_DIR}/Inc)

in the Utils' Cmake file the following piece of code appears:

if (DEFINED FLAG)
   add_compile_options(-DFLAG)
endif(DEFINED FLAG)

I've tried adding the -DFLAG to the cmake args (as seen above), i've tried adding add_definitions, target_compile_definitions, add_compile_definitions to the top of the a.hpp Cmake File but i still get a 'someFunc' is not a member of 'Foo'.