Set libdir to lib64 only if it exists (#632)
Set libdir to lib64 only if it exists (#632)

Modern Linux distros use lib for x86_64 libraries (not lib64). Set
--with-libdir=lib64 only if /usr/lib64 exists. This fixes builds with
--with-openssl=[custom dir] on recent Ubuntu versions.
php-build/php-build,gerardroche/php-build