initial support for static userspace probes via systemtap

This adds a --enable-systemtap configure option, which will then
add static tracepoints to the generated libosmocore binary.

At this point, only two tracepoints are supported: log_start
and log_done.  They can be used to trace the amount of time
a libosmocore-using application spends in potentiall blocking calls to
log to stderr or to files.

Related: OS#4311
Change-Id: I7e1ab664241deb524c9582cbd1bec31af46c747e
diff --git a/Makefile.am b/Makefile.am
index f2a05a9..ab2aad9 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,7 +1,7 @@
 ACLOCAL_AMFLAGS = -I m4
 
 AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include
-SUBDIRS = include src src/vty src/codec src/gsm src/coding src/gb src/ctrl src/sim src/pseudotalloc src/usb utils tests
+SUBDIRS = include src src/vty src/codec src/gsm src/coding src/gb src/ctrl src/sim src/pseudotalloc src/usb utils tapset tests
 
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libosmocore.pc libosmocodec.pc libosmovty.pc libosmogsm.pc \