Patch submitted upstream at https://github.com/librsync/librsync/pull/136
diff -u -r ../librsync-2.0.1/CMakeLists.txt ./CMakeLists.txt
--- ../librsync-2.0.1/CMakeLists.txt	2017-10-17 03:45:57.000000000 +0200
+++ ./CMakeLists.txt	2017-12-23 23:34:02.239823106 +0100
@@ -108,8 +108,13 @@
 check_type_size ( "unsigned short" SIZEOF_UNSIGNED_SHORT )
 
 # Check for printf "%zu" size_t formating support.
-include(CheckCSourceRuns)
-check_c_source_runs("#include <stdio.h>\nint main(){char o[8];sprintf(o, \"%zu\", (size_t)7);return o[0] != '7';}" HAVE_PRINTF_Z)
+if(CMAKE_CROSSCOMPILING)
+  set(HAVE_PRINTF_Z ON)
+  message (STATUS "Cross compiling - assuming printf \"%zu\" size_t formating support")
+else()
+  include(CheckCSourceRuns)
+  check_c_source_runs("#include <stdio.h>\nint main(){char o[8];sprintf(o, \"%zu\", (size_t)7);return o[0] != '7';}" HAVE_PRINTF_Z)
+endif()
 
 include (TestBigEndian)
 TEST_BIG_ENDIAN(WORDS_BIGENDIAN)