NixOS 24.05 - CMake not finding gtkmm3

When I run cmake .. in ./build, I get the output:

`-- Checking for module 'gtkmm3'
--   Package 'gtkmm3', required by 'virtual:world', not found
CMake Error at /nix/store/i78xz11da295kd8ynsjmxvi2r56rbhzw-clion-2024.1.3/clion/bin/cmake/linux/x64/share/cmake-3.28/Modules/FindPkgConfig.cmake:619 (message):
  The following required packages were not found:

   - gtkmm3

Call Stack (most recent call first):
  /nix/store/i78xz11da295kd8ynsjmxvi2r56rbhzw-clion-2024.1.3/clion/bin/cmake/linux/x64/share/cmake-3.28/Modules/FindPkgConfig.cmake:841 (_pkg_check_modules_internal)
  CMakeLists.txt:20 (pkg_check_modules)


-- Configuring incomplete, errors occurred!

I have tried using g++ and ran the following command without any errors. g++ -o ./build/bin/button_example ./src/main.cpp pkg-config --cflags --libs gtkmm-3.0'`

This is my nix flake:

{
  description = "A Nix-flake-based C/C++ development environment with GTK support";

  inputs.nixpkgs.url = "https://flakehub.com/f/NixOS/nixpkgs/0.1.*.tar.gz";

  outputs = { self, nixpkgs }:
    let
      supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
      forEachSupportedSystem = f: nixpkgs.lib.genAttrs supportedSystems (system: f {
        pkgs = import nixpkgs { inherit system; };
      });
    in
    {
      devShells = forEachSupportedSystem ({ pkgs }: {
        default = pkgs.mkShell.override
          {
            # Optionally override stdenv for specific compiler settings:
            # stdenv = pkgs.clangStdenv;
          }
          {
            packages = with pkgs; [
              clang-tools
              cmake
              codespell
              conan
              cppcheck
              doxygen
              gtest
              lcov
              vcpkg
              vcpkg-tool
              gtkmm3
              pkg-config
            ] ++ (if system == "aarch64-darwin" then [ ] else [ gdb ]);
          };
      });
    };
}`

and this is my cmakelists.txt:

# Minimum CMake version
cmake_minimum_required(VERSION 3.21)

# Define the project
project(calculator VERSION 1.0 LANGUAGES CXX)

# Enable C++20 standard
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED YES)
set(CMAKE_CXX_EXTENSIONS NO)


# Set the output directory for binaries
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)

# Include directories
include_directories(${CMAKE_SOURCE_DIR}/src)

# Find gtkmm-3.0 package using pkg-config
find_package(PkgConfig REQUIRED)
pkg_check_modules(GTKMM REQUIRED gtkmm3)

# Include GTKMM directories and link libraries
include_directories(${GTKMM_INCLUDE_DIRS})
link_directories(${GTKMM_LIBRARY_DIRS})
add_definitions(${GTKMM_CFLAGS_OTHER})

# Add the source files
file(GLOB_RECURSE SOURCES "src/*.cpp")

# Create the executable
add_executable(calculator ${SOURCES})

# Link GTKMM libraries
target_link_libraries(calculator ${GTKMM_LIBRARIES})

I have tried using different gtk versions with no luck. I get the same error no matter what, unless I use g++. I'm new-ish both nix and c++, so don't have many idea's on what else to try.