configure.ac: Check for pkg-config

Change-Id: I1a851181a99f2f35ea4ff1b38c7afe27a04e5f18
diff --git a/configure.ac b/configure.ac
index 7d26ced..5c034e5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -49,6 +49,13 @@
 AC_PROG_INSTALL
 AC_PATH_PROG([RM_PROG], [rm])
 
+dnl check for pkg-config (explained in detail in libosmocore/configure.ac)
+AC_PATH_PROG(PKG_CONFIG_INSTALLED, pkg-config, no)
+if test "x$PKG_CONFIG_INSTALLED" = "xno"; then
+        AC_MSG_WARN([You need to install pkg-config])
+fi
+PKG_PROG_PKG_CONFIG([0.20])
+
 AC_LIBTOOL_WIN32_DLL
 AC_ENABLE_SHARED	dnl do build shared libraries
 AC_DISABLE_STATIC	dnl don't build static libraries