mention that --enabel-perf-debug is actually gprof

configure.ac

@@ -824,7 +824,7 @@ fi
 
 AC_MSG_CHECKING(whether performance debugging should be enabled)
 AC_ARG_ENABLE(perf-debug, [AS_HELP_STRING([--enable-perf-debug],
-			[enable performance debugging])],
+			[enable performance debugging through gprof])],
 			enable_perf_debug=$enableval, enable_perf_debug=no)
 AC_MSG_RESULT($enable_perf_debug)
 AC_SUBST(STARPU_PERF_DEBUG, $enable_perf_debug)

doc/starpu.texi

@@ -2564,7 +2564,7 @@ Specify the location of the Gordon SDK.
 @subsubsection @code{--enable-perf-debug}
 @table @asis
 @item @emph{Description}:
-Enable performance debugging.
+Enable performance debugging through gprof.
 @end table
 
 @node --enable-model-debug