Add documentation string for LDFLAGS_SL in configure --help output.

configure

@@ -1431,7 +1431,7 @@ Some influential environment variables:
   CPPFLAGS    C/C++/Objective C preprocessor flags, e.g. -I<include dir> if
               you have headers in a nonstandard directory <include dir>
   CPP         C preprocessor
-  LDFLAGS_SL
+  LDFLAGS_SL  linker flags for shared library linking
   DOCBOOKSTYLE
               location of DocBook stylesheets

configure.in

 dnl Process this file with autoconf to produce a configure script.
-dnl $PostgreSQL: pgsql/configure.in,v 1.596 2009/05/15 02:18:27 scrappy Exp $
+dnl $PostgreSQL: pgsql/configure.in,v 1.597 2009/05/19 22:32:41 petere Exp $
 dnl
 dnl Developers, please strive to achieve this order:
 dnl
@@ -794,7 +794,7 @@ AC_SUBST(ELF_SYS)
 CPPFLAGS="$CPPFLAGS $INCLUDES"
 LDFLAGS="$LDFLAGS $LIBDIRS"
 
-AC_ARG_VAR(LDFLAGS_SL)
+AC_ARG_VAR(LDFLAGS_SL, [linker flags for shared library linking])
 
 PGAC_PROG_LD
 AC_SUBST(LD)