build: remove unused all_includes and use AM_CPPFLAGS
Preprocessor flags are best placed in AM_CPPFLAGS. Remove use of the
unused all_includes variable, which is never set.
diff --git a/src/Makefile.am b/src/Makefile.am
index f2b1b83..65494a5 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -2,7 +2,8 @@
# Please read Chapter 6 "Library interface versions" of the libtool documentation before making any modification
LIBVERSION=5:0:0
-AM_CFLAGS = -Wall $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)/include
+AM_CPPFLAGS = -I$(top_srcdir)/include -I$(top_builddir)/include
+AM_CFLAGS = -Wall
lib_LTLIBRARIES = libosmocore.la