cmake: Add --no-undefined linker option

CMakeLists.txt

@@ -269,6 +269,11 @@ if(SECP256K1_BUILD_CTIME_TESTS)
   unset(msan_enabled)
 endif()
 
+if(NOT SECP256K1_USE_EXTERNAL_DEFAULT_CALLBACKS)
+  include(TryAppendLinkerFlags)
+  try_append_linker_flags(-Wl,--no-undefined)
+endif()
+
 include(CTest)
 # We do not use CTest's BUILD_TESTING because a single toggle for all tests is too coarse for our needs.
 mark_as_advanced(BUILD_TESTING)

cmake/TryAppendLinkerFlags.cmake

@@ -0,0 +1,32 @@
+include_guard(GLOBAL)
+
+include(CheckCSourceCompiles)
+
+function(secp256k1_check_linker_flags_internal flags output)
+  string(MAKE_C_IDENTIFIER "${flags}" result)
+  string(TOUPPER "${result}" result)
+  set(result "LINKER_SUPPORTS_${result}")
+  set(CMAKE_REQUIRED_FLAGS "${flags}")
+  if(MSVC)
+    string(APPEND " /WX")
+  elseif(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+    string(APPEND " -Wl,-fatal_warnings")
+  else()
+    string(APPEND " -Wl,--fatal-warnings")
+  endif()
+
+  # This ensures running a linker.
+  set(CMAKE_TRY_COMPILE_TARGET_TYPE EXECUTABLE)
+  check_c_source_compiles("int main() { return 0; }" ${result})
+
+  set(${output} ${${result}} PARENT_SCOPE)
+endfunction()
+
+# Append flags to the LINK_OPTIONS directory property if a linker accepts them.
+macro(try_append_linker_flags)
+  secp256k1_check_linker_flags_internal("${ARGV}" result)
+  if(result)
+    add_link_options(${ARGV})
+  endif()
+  unset(result)
+endmacro()