Please describe the problem:
The ORBit2 configure script fails to parse if the pkg.m4 from pkg-config >=
0.16.0 is picked up.

Steps to reproduce:
1. install pkg-config 0.16.0
2. check out ORBit2 from CVS
3. run ORBit2's autogen.sh script


Actual results:
The following error is printed:
configure.in:118: error: possibly undefined macro: PKG_CONFIG_MIN_VERSION
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.


Expected results:
ORBit2 is configured successfully.

Does this happen every time?
yes

Other information:
The pkg.m4 macro file in pkg-config 0.16.0 contains the following code:
  m4_pattern_forbid([^_?PKG_[A-Z_]+$])
  m4_pattern_allow([^PKG_CONFIG(_PATH)?$])

In effect, this reserves the PKG_* and _PKG_* namespaces for its use in shell
code (except for $PKG_CONFIG and $PKG_CONFIG_PATH).  This feature is done to
detect bugs where unknown macros are used in a configure script and end up
unexpanded.

ORBit2's configure script defines a shell variable $PKG_CONFIG_MIN_VERSION,
which causes the error.  The two fixes are:
 1. rename the variable (eg. $xPKG_CONFIG_MIN_VERSION)
 2. add the line m4_pattern_allow([^PKG_CONFIG_MIN_VERSION$])

The first option is probably a better choice.