Fix default lib for x86_64 Debian/Ubuntu

The distribution provided architecture specific RPM macro files
for x86_64 on Debian/Ubuntu specify the wrong default install
location.  When building x86_64 deb packages override _lib
with the correct location.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #7083

config/zfs-build.m4

@@ -192,6 +192,13 @@ AC_DEFUN([ZFS_AC_RPM], [
 	RPM_DEFINE_UTIL+=' $(DEFINE_INITRAMFS)'
 	RPM_DEFINE_UTIL+=' $(DEFINE_SYSTEMD)'
 
+	dnl # Override default lib directory on x86_64 Debian/Ubuntu systems.
+	dnl # The provided /usr/lib/rpm/platform/x86_64-linux/macros file
+	dnl # defaults to 'lib64' instead of 'lib/x86_64-linux-gnu'.
+	AS_IF([test "$DEFAULT_PACKAGE" = "deb" -a "$target_cpu" = "x86_64" ], [
+		RPM_DEFINE_UTIL+=' --define "_lib lib/x86_64-linux-gnu"'
+	])
+
 	RPM_DEFINE_KMOD='--define "kernels $(LINUX_VERSION)"'
 	RPM_DEFINE_KMOD+=' --define "require_spldir $(SPL)"'
 	RPM_DEFINE_KMOD+=' --define "require_splobj $(SPL_OBJ)"'