- Use tabs instead of spaces for indentation

- Proper m4 quoting of parameters

- Fix broken AC_ARG_* handling:
  --without-foo/--disable-foo set withval="no"/enableval="no"
  and the third parameter of the AC_ARG_* macros is always
  "user provided an --with*-foo/--*able-foo option" _NOT_ "yes",
  while the fourth parameter is "user didn't provide this option"
  (= the default if neither --with-foo/--enable-foo nor --without-foo/--disable-foo
     has been set)

- Drop FTDM_CFLAGS in MY_CFLAGS (what was that used for anyway?)

- Drop MYLIB

- Drop SOLINK (unused)

- Rename MY_CFLAGS to FTDM_CFLAGS

- Rename all AM_CONDITION variables to HAVE_* for (optional) dependencies
  and WANT_* for features

- Reorder mod_LTLIBRARIES lines

- Move some things in configure.ac around to group them

- Add -shared to modules LDFLAGS (we don't need "ftmod_foo.a" static modules, do we?)

- Import ax_compiler_vendor.m4, drop the macro definition from configure.ac

- Add AUTOMAKE_OPTS to Makefile.am and define FreeTDM as a foreign (= non-GNU) project

- Add ACLOCAL_FLAGS to Makefile.am and set AC_CONFIG_MACRO_DIR in configure.ac
  custom m4 macros are kept in the build subdirectory

- Enable silent rules for automake-1.11 and newer (use make V=1 to get verbose output)

- configure option --with-modinstdir requires a value now and quits with an error message
  if called without one (e.g. "./configure --without-modinstdir" or "./configure --with-modinstdir")

- Add configuration summary screen and category headers for modules in configure output