Move all the paths into a new path.h to centralize them.
Rather than using the #define for path names, indirect through a char *
variable that could change for different executable types detected.
Use a macro to create the names for the library path names. This will
allow later substitution at run time instead of compile time of the
environment variable name prefix.
Create a generalized exec hook that different architectures can hook
into if they need to, but default to no action.
If md_exec_hook is defined, provide a way to create the strings
for the environment variables we look up at runtime. Otherwise,
there's no way they will change, optimize it at compile time.