Improve build script error checking.

This commit adds a check to make sure pkg-config is present on the
computer building libzmq.
This particular line was stolen from the debian mailing list.
See https://lists.debian.org/debian-dpkg/2011/04/msg00054.html

configure.ac

@@ -49,6 +49,7 @@ AM_PROG_CC_C_O
 AC_PROG_SED
 AC_PROG_AWK
 PKG_PROG_PKG_CONFIG
+m4_pattern_forbid([^PKG_[A-Z_]+$], [missing some pkg-config macros (pkg-config package)])
 
 # Libtool configuration for different targets. See acinclude.m4
 AC_ARG_VAR([XMLTO], [Path to xmlto command])