Remove trailing whitespace from m4 file

m4/compiler-flags.m4

@@ -25,7 +25,7 @@ AC_DEFUN([gl_COMPILER_FLAGS],
    AC_SUBST(COMPILER_FLAGS)
    ac_save_CFLAGS="$CFLAGS"
    dnl Some flags are dependant, so we set all previously checked
-   dnl flags when testing. Except for -Werror which we have to 
+   dnl flags when testing. Except for -Werror which we have to
    dnl check on its own, because some of our compiler flags cause
    dnl warnings from the autoconf test program!
    if test "$1" = "-Werror" ; then