[channels,rdpear] explicitly abort if Heimdal detected

The RDPEAR channel is currently only tested against MIT kerberos. It
does not even compile with Heimdal, so bail out early during configure.

channels/rdpear/CMakeLists.txt

@@ -21,6 +21,10 @@ if(NOT IOS AND NOT WIN32 AND NOT ANDROID)
   if(KRB5_FOUND)
     define_channel("rdpear")
 
+    if(KRB5_FLAVOUR STREQUAL "Heimdal")
+      message(FATAL_ERROR "krb5 implementation Heimdal not supported with -DCHANNEL_RDPEAR=ON")
+    endif()
+
     include_directories(common)
 
     if(WITH_CLIENT_CHANNELS OR WITH_SERVER_CHANNELS)