make compiling without autotools easier

[darealshinji]

I sometimes like to download the latest snapshot of patchelf.cc when I need it and compile it in a single command like this: g++ -DPAGESIZE=4096 -DPACKAGE_STRING=\"\" -std=c++11 -o patchelf patchelf.cc
It would be convenient if PAGESIZE and PACKAGE_STRING did fall back to some default values if not specified through CXXFLAGS, in such a way that it of course won't break the autotools build system. I'm thinking of adding something like this:

#ifndef PAGESIZE
#  define PAGESIZE 4096
#endif
#ifndef PACKAGE_STRING
#  define PACKAGE_STRING "unknown"
#endif

[Ericson2314]

This was fixed in #299