--- CMakeLists.txt.orig	2021-04-07 12:23:46 UTC
+++ CMakeLists.txt
@@ -408,6 +408,10 @@ test_big_endian (HAVE_BIGENDIAN)
 if (HAVE_BIGENDIAN)
     add_definitions (-DHAVE_BIGENDIAN)
 endif ()
+# CMake 3.20 doesn't do the tests internally in TestBigEndian
+# that end up setting HAVE_STDINT_H and similar.
+include(CheckTypeSize)
+CHECK_TYPE_SIZE("unsigned short" CMAKE_SIZEOF_UNSIGNED_SHORT LANGUAGE C)
 
 # Force certain components to always be built as release
 macro (force_release_build)