Configure #1: rename configure.in to configure.ac.

This commit and the following commits marked "Configure #x:" will
update the autoconf/configure system to a more future-proof state
while maintaining compatibility with autoconf 2.50:
  - rename configure.in to configure.ac (this commit)
  - replace deprecated configure macros with recommended ones
  - simplify some configure macros (e.g. CHECK_HEADERS instead of CHECH_HEADER)
  - improve indenting (clarify macro arguments)
  - better (combined) header and library checks
  - and some more small changes.


git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@11994 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

Makefile

@@ -101,7 +101,7 @@ makeinclude: configure configh.in makeinclude.in config.guess config.sub
 	touch config.h
 	chmod +x fltk-config
 
-configure: configure.in
+configure: configure.ac
 	autoconf
 
 config.guess config.sub: