some doc for --enable-fixed-point and --enable-floating-point

configure.ac

@@ -154,15 +154,15 @@ has_float_approx=no
 
 ac_enable_fixed="no";
 ac_enable_float="yes";
-AC_ARG_ENABLE(fixed-point, [  --enable-fixed-point       compile without floating point],
+AC_ARG_ENABLE(fixed-point, [  --enable-fixed-point       compile without floating point (for machines without a fast enough FPU)],
 [if test "$enableval" = yes; then
   ac_enable_fixed="yes";
   ac_enable_float="no";
-  AC_DEFINE([FIXED_POINT], [1], [Compile as fixed-point])
+  AC_DEFINE([FIXED_POINT], [1], [Compile as fixed-point (for machines without a fast enough FPU)])
 else
-  AC_DEFINE([FLOATING_POINT], , [Compile as floating-point])
+  AC_DEFINE([FLOATING_POINT], , [Compile as floating-point (for machines with a fast enough FPU)])
 fi],
-AC_DEFINE([FLOATING_POINT], , [Compile as floating-point]))
+AC_DEFINE([FLOATING_POINT], , [Compile as floating-point (for machines with a fast enough FPU)]))
 
 ac_enable_fixed_debug="no"
 AC_ARG_ENABLE(fixed-point-debug, [  --enable-fixed-point-debug debug fixed-point implementation],