Avoid complaining from strlen

`

utf8proc/utils.cmake

@@ -0,0 +1,20 @@
+
+function (disallow_intree_builds)
+  # Adapted from LLVM's toplevel CMakeLists.txt file
+  if( CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR AND NOT MSVC_IDE )
+    message(FATAL_ERROR "
+      In-source builds are not allowed. CMake would overwrite the
+      makefiles distributed with utf8proc. Please create a directory
+      and run cmake from there. Building in a subdirectory is
+      fine, e.g.:
+      
+        mkdir build
+        cd build
+        cmake ..
+      
+      This process created the file `CMakeCache.txt' and the
+      directory `CMakeFiles'. Please delete them.
+      
+      ")
+  endif()
+endfunction()