Makefile: Fix cross-compilation for 32-bit targets

Determining the correct library installation path (lib vs. lib64)
using uname(1) breaks in cross compilation scenarios where word widths
differ between the host and target system.

Instead, source the information from the compilers '-dumpmachine'
option (supported by both GCC and Clang).

We call this the "host" architecture, using the same nomenclature as
Autotools (--host configure option).

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>

src/Makefile

@@ -77,7 +77,8 @@ INSTALL = install
 
 DESTDIR ?=
 
-ifeq ($(filter-out %64 %64be %64eb %64le %64el s390x, $(shell uname -m)),)
+HOSTARCH = $(firstword $(subst -, ,$(shell $(CC) -dumpmachine)))
+ifeq ($(filter-out %64 %64be %64eb %64le %64el s390x, $(HOSTARCH)),)
 	LIBSUBDIR := lib64
 else
 	LIBSUBDIR := lib