Allow specifying sysmocom headers explicitly

The headers for LC1.5 are specified explicitly. Add corresponding option
to specify sysmoBTS headers location and use it in jenkins build. While
at it, unify header fixup code with the one used in OsmoBTS.

Change-Id: I5248e8b389fd240b4d5a0bcf6c954d6115262462
diff --git a/src/Makefile.am b/src/Makefile.am
index a6e98e5..7d2a62e 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -103,7 +103,7 @@
 osmo_pcu_SOURCES = pcu_main.cpp
 
 if ENABLE_SYSMODSP
-AM_CPPFLAGS += -I$(srcdir)/osmo-bts-sysmo
+AM_CPPFLAGS += -I$(srcdir)/osmo-bts-sysmo -I$(SYSMOBTS_INCDIR)
 
 EXTRA_DIST = \
 	osmo-bts-sysmo/sysmo_l1_if.c \