build: Use AM_CPPFLAGS in Makefile.am

Since automake 1.13 INCLUDES is depricates and causes a warning

Inspired from similar patches by Alexander Huemer for other osmocom
projects

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/Makefile.am b/Makefile.am
index b5e0d59..d1638ec 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,7 +1,7 @@
 AUTOMAKE_OPTIONS = foreign dist-bzip2 1.6
 ACLOCAL_AMFLAGS = -I m4
 
-INCLUDES = $(all_includes) -I$(top_srcdir)/include
+AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include
 
 SUBDIRS = include
 
diff --git a/libgsmhr/Makefile.am b/libgsmhr/Makefile.am
index ab49a1f..07390f9 100644
--- a/libgsmhr/Makefile.am
+++ b/libgsmhr/Makefile.am
@@ -1,4 +1,4 @@
-INCLUDES = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)
+AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)
 AM_CFLAGS = -fPIC -Wall ${SYMBOL_VISIBILITY}
 
 LIBVERSION=0:0:0
diff --git a/src/Makefile.am b/src/Makefile.am
index c17dd3f..818f215 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -1,4 +1,4 @@
-INCLUDES = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)
+AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)
 AM_CFLAGS=-Wall $(LIBOSMOCODEC_CFLAGS) $(LIBOSMOCORE_CFLAGS) \
 	  ${OPENCORE_AMRNB_CFLAGS}
 AM_LDFLAGS=$(LIBOSMOCODEC_LIBS) $(LIBOSMOCORE_LIBS) \