configure: misc modernizations

- require autoconf version 2.69
  was released in 2012 and one still can configure and build on older
  systems (just not generate the configure script)
- use modern C99 compiler check
- drop obsolete checks: AC_C_CONST, AC_FUNC_CLOSEDIR_VOID, AC_FUNC_STAT
- drop AC_HEADER_STDBOOL in favor of C99 compatibility

Makefile.am

@@ -3,8 +3,16 @@ AUTOMAKE_OPTIONS = subdir-objects
 bin_PROGRAMS = htop
 
 dist_man_MANS = htop.1
-EXTRA_DIST = $(dist_man_MANS) htop.desktop htop.png htop.svg \
-install-sh autogen.sh missing
+EXTRA_DIST = \
+	$(dist_man_MANS) \
+	autogen.sh \
+	htop.desktop \
+	htop.png \
+	htop.svg \
+	build-aux/compile \
+	build-aux/depcomp \
+	build-aux/install-sh \
+	build-aux/missing
 applicationsdir = $(datadir)/applications
 applications_DATA = htop.desktop
 pixmapdir = $(datadir)/pixmaps