CMakeLists.txt: Add detection of uint64_t (#52)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index f8716a3..54a13c0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -59,6 +59,12 @@
 check_include_files(sys/types.h INCLUDE_SYS_TYPES_H)
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
+set(SIZE16 int16_t)
+set(USIZE16 uint16_t)
+set(SIZE32 int32_t)
+set(USIZE32 uint32_t)
+set(SIZE64 int64_t)
+set(USIZE64 uint64_t)
 
 include(CheckSizes)