And actually use the values found by cmake

CMakeLists.txt

@@ -80,3 +80,5 @@ target_link_libraries(test
   irc
   bridge
   utils)
+
+CONFIGURE_FILE(config.h.cmake config.h @ONLY)

config.h.cmake

@@ -0,0 +1 @@
+#cmakedefine ICONV_SECOND_ARGUMENT_IS_CONST

src/utils/encoding.cpp

@@ -7,6 +7,8 @@
 #include <string.h>
 #include <iconv.h>
 
+#include "config.h"
+
 /**
  * The UTF-8-encoded character used as a place holder when a character conversion fails.
  * This is U+FFFD � "replacement character"