config.site for vendor libs on Fedora x86_64 - x86-64

Question

I'm having trouble building a few Autotool-based libraries on Fedora 26, x86_64. The 64-bit Fedora puts third party and vendor libraries in /usr/local/lib64. Ubuntu 17 uses /usr/local/lib so the same projects build OK.
I've been using --libdir=/usr/local/lib64 but three libraries resist it. I lack a config.site for /usr/local so I am trying to add one. The Autoconf manual on Site Defaults is a tad bit confusing to me when it discusses usr/local's config.site. It says:
[discussion of /usr version of confg.site] ...
Likewise, on platforms where 64-bit libraries are built by default,
then installed in /usr/local/lib64 instead of /usr/local/lib, it is
appropriate to install /usr/local/share/config.site:
# /usr/local/share/config.site for platforms that prefer
# the directory /usr/local/lib64 over /usr/local/lib.
test "$libdir" = '${exec_prefix}/lib' && libdir='${exec_prefix}/lib64'
The problem I am having is, is the modification above appended to the /usr/local version of config.site? Or does it replace an existing block of code? Or can I just copy it where it belongs without modification?
Or maybe, what does a cat of /usr/local/share/config.site look like?
Here is the config.site for /usr. Its not clear to me if it needs modification or how to modify it.
$ cat /usr/share/config.site
# This is the config.site file to satisfy FHS defaults when installing below
# /usr.
#
# You may override this file by your config.site using the CONFIG_SITE env
# variable.
#
# Note: This file includes also RHEL/Fedora fix for installing libraries into
# "/lib/lib64" on 64bit systems.
if test -n "$host"; then
# skip when cross-compiling
return 0
fi
if test "$prefix" = /usr \
|| { test "$prefix" = NONE && test "$ac_default_prefix" = /usr ; }
then
test "$sysconfdir" = '${prefix}/etc' && sysconfdir=/etc
test "$sharedstatedir" = '${prefix}/com' && sharedstatedir=/var
test "$localstatedir" = '${prefix}/var' && localstatedir=/var
ARCH=`uname -m`
for i in x86_64 ppc64 s390x aarch64; do
if test $ARCH = $i; then
test "$libdir" = '${exec_prefix}/lib' && libdir='${exec_prefix}/lib64'
break
fi
done
fi

Answer 1

config.site for vendor libs on Fedora x86_64
This answers the question of what config.site looks like for /usr/local/share/config.site. It does not answer the question of why --libdir=/usr/local/lib64 fails to set the directory, as #John Bollinger pointed out in the comments.
The /usr/local/share/config.site is wrong. Though it was copied from Fedora's config.site and placed in /usr/local/share, the prefix directories are wrong. The prefix test should use /usr/local and not /usr.
Below is the corrected one.
$ cat /usr/local/share/config.site
...
if test -n "$host"; then
# skip when cross-compiling
return 0
fi
if test "$prefix" = /usr/local \
|| { test "$prefix" = NONE && test "$ac_default_prefix" = /usr/local ; }
then
test "$sysconfdir" = '${prefix}/etc' && sysconfdir=/etc
test "$sharedstatedir" = '${prefix}/com' && sharedstatedir=/var
test "$localstatedir" = '${prefix}/var' && localstatedir=/var
ARCH=`uname -m`
for i in x86_64 ppc64 s390x aarch64; do
if test $ARCH = $i; then
test "$libdir" = '${exec_prefix}/lib' && libdir='${exec_prefix}/lib64'
break
fi
done
fi
I'm not sure if these are correct, however. They were not modified.
test "$sysconfdir" = '${prefix}/etc' && sysconfdir=/etc
test "$sharedstatedir" = '${prefix}/com' && sharedstatedir=/var
test "$localstatedir" = '${prefix}/var' && localstatedir=/var
Now, the next question is, why Fedora's /usr/share/config.site is not handling prefix=/usr/local properly. That's an open question at Issue 1510073 : Autoconf does not honor libdir in config.site for "libdir=#libdir#" in *.pc file, which has been closed as NOT A BUG.