Merge branch 'refs/heads/t/lopsub'

Three simple patches for the build system, all related to lopsub.

Cooking for three weeks.

* refs/heads/t/lopsub:
  build: Echo "M4" when running an m4 command.
  build: Run lopsubgen with absolute path.
  build: Improve lopsub error diagnostics.

diff --combined configure.ac
index c2cb47e6f993c6cbbad343208ce634af8bf80173,0cc0df57fbe0beebb95997891cff39f3fd6d7639..811364b19b4aded3c0d5f090ef16094e8a5162ea
--- 1/configure.ac
--- 2/configure.ac
+++ b/configure.ac
@@@ -6,6 -6,7 +6,6 @@@ AC_CONFIG_HEADERS([config.h]
  
  AC_CONFIG_FILES([Makefile])
  AC_DEFUN([add_dot_o],[$(for i in $@; do printf "$i.o "; done)])
 -AC_DEFUN([add_cmdline],[$(for i in $@; do printf "${i}.cmdline "; done)])
  AC_DEFUN([LIB_ARG_WITH], [
        AC_ARG_WITH($1-headers, [AS_HELP_STRING(--with-$1-headers=dir,
                [look for $1 headers in dir])])
@@@ -54,10 -55,6 +54,6 @@@ AC_PATH_PROG([M4], [m4]
  test -z "$M4" && AC_MSG_ERROR(
        [The m4 macro processor is required to build this package])
  
- AC_PATH_PROG([lopsubgen], [lopsubgen])
- test -z "$lopsubgen" && AC_MSG_ERROR(
-       [lopsubgen is required to build this package])
- 
  AC_PROG_CC
  AC_PROG_CPP
  
@@@ -71,9 -68,11 +67,11 @@@ AC_CHECK_LIB([osl], [osl_open_table], [
  LIB_SUBST_FLAGS(osl)
  UNSTASH_FLAGS
  ######################################################################## lopsub
+ HAVE_LOPSUB=yes
+ AC_PATH_PROG([LOPSUBGEN], [lopsubgen])
+ test -z "$LOPSUBGEN" && HAVE_LOPSUB=no
  STASH_FLAGS
  LIB_ARG_WITH([lopsub], [-llopsub])
- HAVE_LOPSUB=yes
  AC_CHECK_HEADER(lopsub.h, [], [HAVE_LOPSUB=no])
  AC_CHECK_LIB([lopsub], [lls_merge], [], [HAVE_LOPSUB=no])
  if test $HAVE_LOPSUB = no; then AC_MSG_ERROR([
@@@ -196,7 -195,7 +194,7 @@@ AC_LINK_IFELSE([AC_LANG_PROGRAM([
  ]])],[have_ip_mreqn=yes],[have_ip_mreqn=no])
  AC_MSG_RESULT($have_ip_mreqn)
  if test ${have_ip_mreqn} = yes; then
 -      AC_DEFINE(HAVE_IP_MREQN, 1, define to 1 you have struct ip_mreqn)
 +      AC_DEFINE(HAVE_IP_MREQN, 1, define to 1 if you have struct ip_mreqn)
  fi
  ########################################################################### ogg
  STASH_FLAGS