build: Add a proper pkgconfig libgtp.pc for the GTP library

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/Makefile.am b/Makefile.am
index 581e1bd..960ca07 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,2 +1,5 @@
 ## Process this file with automake to produce Makefile.in
 SUBDIRS = lib gtp ggsn sgsnemu doc
+
+pkgconfigdir = $(libdir)/pkgconfig
+pkgconfig_DATA = libgtp.pc
diff --git a/configure.in b/configure.in
index 36a5cbe..3aa6909 100644
--- a/configure.in
+++ b/configure.in
@@ -115,5 +115,6 @@
                  sgsnemu/Makefile
                  src/Makefile
                  tests/Makefile
+                 libgtp.pc
                  openggsn.spec])
 AC_OUTPUT
diff --git a/libgtp.pc.in b/libgtp.pc.in
new file mode 100644
index 0000000..2201608
--- /dev/null
+++ b/libgtp.pc.in
@@ -0,0 +1,11 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: OpenGGSN STP Library
+Description: C Utility Library
+Version: @VERSION@
+Libs: -L${libdir} -lgtp
+Cflags: -I${includedir}/
+