build: only use HOSTNAME_PRG if HOSTNAME is undefined (#22288)

Allow specifying the HOSTNAME variable by defining it with -D when
invoking CMake, instead of overriding it with the output of
HOSTNAME_RPG.

cmake.config/CMakeLists.txt

@@ -188,7 +188,7 @@ elseif (NOT DEFINED USERNAME AND EXISTS ${WHOAMI_PRG})
 endif()
 if (DEFINED ENV{HOSTNAME})
   set(HOSTNAME $ENV{HOSTNAME})
-elseif (EXISTS ${HOSTNAME_PRG})
+elseif (NOT DEFINED HOSTNAME AND EXISTS ${HOSTNAME_PRG})
   execute_process(COMMAND ${HOSTNAME_PRG}
     OUTPUT_STRIP_TRAILING_WHITESPACE
     OUTPUT_VARIABLE HOSTNAME)