Blitz header file notes

1) Some matrix headers are included in this release, but only because
the Benchmark class needs them.  Their design is not yet stable,
and they are untested, so use them at your own peril.

2) A compiler-specific header file with configuration settings is included
from the master header file <blitz/bzconfig.h>.  The compiler-specific 
header file is selected on the basis of preprocessor symbols.  Since some
C++ compilers (notably the IBM XL compiler for Darwin and for AIX systems)
do not define a unique preprocessor symbol, we have added a -D option in
the autoconf macro AC_CXX_FLAGS_PRESET (see file m4/ac_cxx_flags_preset.m4).
Thus, we use the option -D__APPLE with xlc++ and -D__IBM with xlC.
Please note that any user code must also be compiled with the same -D option
in order to be compiled successfully with one of these compilers.