[mlir][cmake] Enable -Wundef.

This is another follow-up of llvm#83004, which fixed a bug due to some
macros not being defined in some situations. By raising warnings on
undefined macros, this kind of bug is less likely to be introduced.
Similar to llvm#83004, the fix is probably adding an include to
`mlir-config.h` (and potentially defining the macro there).

Author: ingomueller-net

mlir/CMakeLists.txt

@@ -70,6 +70,12 @@ endif()
 check_c_compiler_flag("-Werror=implicit-function-declaration" C_SUPPORTS_WERROR_IMPLICIT_FUNCTION_DECLARATION)
 append_if(C_SUPPORTS_WERROR_IMPLICIT_FUNCTION_DECLARATION "-Werror=implicit-function-declaration" CMAKE_C_FLAGS)
 
+# Warn on undefined macros. This is often an indication that an include to
+# `mlir-config.h` or similar is missing.
+check_c_compiler_flag("-Wundef" C_SUPPORTS_WUNDEF)
+append_if(C_SUPPORTS_WUNDEF "-Wundef" CMAKE_C_FLAGS)
+append_if(C_SUPPORTS_WUNDEF "-Wundef" CMAKE_CXX_FLAGS)
+
 # Forbid mismatch between declaration and definition for class vs struct. This is
 # harmless on Unix systems, but it'll be a ticking bomb for MSVC/Windows systems
 # where it creeps into the ABI.