gsm/Makefile.am: use proper top_builddir/top_srcdir paths

This fixes the conv*gen.c targets when building in a different directory
than the source tree.

Notably, building in a different dir worked when the generated sources were
already present from a previous build inside the source directory.

Change-Id: I5a9b780ad4ba607ea39854dcf7207ed05f5447bc
diff --git a/src/gsm/Makefile.am b/src/gsm/Makefile.am
index a2f2524..2071413 100644
--- a/src/gsm/Makefile.am
+++ b/src/gsm/Makefile.am
@@ -29,7 +29,7 @@
 			milenage/milenage.c gan.c ipa.c gsm0341.c apn.c \
 			gsup.c gprs_gea.c
 libgsmint_la_LDFLAGS = -no-undefined
-libgsmint_la_LIBADD = ../libosmocore.la
+libgsmint_la_LIBADD = $(top_builddir)/src/libosmocore.la
 
 libosmogsm_la_SOURCES =
 libosmogsm_la_LDFLAGS = $(LTLDFLAGS_OSMOGSM) -version-info $(LIBVERSION) -no-undefined $(TALLOC_LIBS)
@@ -37,5 +37,5 @@
 
 EXTRA_DIST = libosmogsm.map
 
-conv%gen.c: ../../utils/conv_gen.py
-	$(AM_V_GEN)python2 ../../utils/conv_gen.py
+conv%gen.c: $(top_srcdir)/utils/conv_gen.py
+	$(AM_V_GEN)python2 $(top_srcdir)/utils/conv_gen.py