Use AM_MISSING_PROG instead of hardcoding missing

Makefile.am

@@ -80,7 +80,7 @@ EXTRA_DIST += ChangeLog
 ChangeLog: $(srcdir)/ChangeLog
 $(srcdir)/ChangeLog:
 	if test -d "$(srcdir)/.git"; then \
-	  (GIT_DIR=$(top_srcdir)/.git ./missing --run git log --stat) | fmt --split-only > $@.tmp \
+	  (GIT_DIR=$(top_srcdir)/.git $(GIT) log --stat) | fmt --split-only > $@.tmp \
 	  && mv -f $@.tmp $@ \
 	  || ($(RM) $@.tmp; \
 	      echo Failed to generate ChangeLog, your ChangeLog may be outdated >&2; \

configure.ac

@@ -70,6 +70,7 @@ AC_PROG_MAKE_SET
 PKG_PROG_PKG_CONFIG
 m4_ifdef([PKG_INSTALLDIR], [PKG_INSTALLDIR], AC_SUBST([pkgconfigdir], ${libdir}/pkgconfig))
 
+AM_MISSING_PROG([GIT], [git])
 AM_MISSING_PROG([GPERF], [gperf])
 
 AC_MSG_CHECKING([for RM macro])