Use --modversion instead of --exists to check library is installed

pkg-config --exists also checks dependencies are present on macOS, and
due to some packaging issues with upstream libarchive this is breaking
the build for cheribsd and freebsd.

Closes CTSRD-CHERI#340.

See Homebrew/homebrew-core#120526 for
tracking of this issue as it relates to macOS.

pycheribuild/projects/simple_project.py

@@ -821,7 +821,7 @@ def check_required_pkg_config(self, package: str, instructions: InstallInstructi
             return  # already checked
         self._validate_cheribuild_target_for_system_deps(instructions.cheribuild_target)
         try:
-            self.run_cmd(["pkg-config", "--exists", package])
+            self.run_cmd(["pkg-config", "--modversion", package], capture_output=True)
         except subprocess.CalledProcessError as e:
             self.dependency_error("Required library", package, "is missing:", e, install_instructions=instructions,
                                   cheribuild_target=instructions.cheribuild_target,