autoconf: More informative errors when pkg-config macros undefined

Added a check that PKG_PROG_PKG_CONFIG is defined before calling it so
the user can see a proper error. Without it, the generated configure
script will just pass on to the next statement and bomb with a strange
shell syntax error.

configure.ac

@@ -28,6 +28,12 @@ AC_PROG_CXX
 AC_PATH_PROG(MAKE, make)
 AC_PATH_PROG(MKDEP, makedepend)
 AC_PATH_PROG(SED, sed)
+
+dnl Make sure the pkg-config macros are defined
+m4_ifdef([PKG_PROG_PKG_CONFIG],,[
+    AC_MSG_ERROR([The pkg-config autoconf macros are not defined.
+                  Did you run 'make configure'?])]
+)
 PKG_PROG_PKG_CONFIG()
 
 dnl LIB_DIR - library basename