Fix m4 warnings from autotools

Makefile.am

@@ -1,4 +1,6 @@
 EXTRA_DIST = MIGRATION README.rst libmodbus.spec
+ACLOCAL_AMFLAGS = -I m4 ${ACLOCAL_FLAGS}
+
 SUBDIRS = src doc tests
 
 pkgconfigdir = $(libdir)/pkgconfig

configure.ac

@@ -28,6 +28,7 @@ AC_CONFIG_HEADERS([config.h tests/unit-test.h])
 AM_INIT_AUTOMAKE([foreign])
 # enable nice build output on automake1.11
 m4_ifdef([AM_SILENT_RULES],[AM_SILENT_RULES([yes])])
+AC_CONFIG_MACRO_DIR([m4])
 
 LIBMODBUS_VERSION_MAJOR=libmodbus_version_major
 LIBMODBUS_VERSION_MINOR=libmodbus_version_minor

m4/.gitignore

@@ -0,0 +1,5 @@
+libtool.m4
+ltoptions.m4
+ltsugar.m4
+ltversion.m4
+lt~obsolete.m4