CentOS - Aug 2018 - Packaging a library for 64-bit or multilib

How does "make install" with autotools decide which directory
libraries
should go in?

I haven't packaged anything since 32-bit (ie. a few years ago), and I now 
need to re-package a new version of a library I'd used before with CentOS 6 
32-bit on CentOS 7 64-bit.

The library is pxlib (http://pxlib.sourceforge.net/), which reads and 
writes old Borland Paradox database files.

I've managed to fix 64-bit issues in the C sources. When I go to package 
the RPM, it runs configure and make and "make DESTDIR=${RPM_BUILD_ROOT} 
install". The install step puts the resulting libraries in /usr/lib, not 
/usr/lib64 where the %files step looks.

I'm not familiar with autotools, so I don't know where to look to fix
the
install step to put the libraries in the correct directory. How does 
make/configure or whatever figure that out? Do I need to change the %prep 
step to run autotools to write the correct library directory for configure, 
or does configure need to be hacked to write the correct directory into the 
Makefiles? Maybe configure needs to be regenerated with a more recent 
autotools?

Found the problem. The spec file was using "./configure". Replacing
that
with "%configure" caused it to use the correct libdir variable.

It also revealed some more warnings at compile time, so I need to go poke 
through those.