Detect target platform consistently with $(CC) -dumpmachine

We already used this for Windows, now use it for all platforms to
simplify cross compilation in general.

Makefile.pdlibbuilder

@@ -430,38 +430,33 @@ endif
 
 
 ################################################################################
-### platform detection #########################################################
+### target platform detection ##################################################
 ################################################################################
 
 
+# Let (native or cross-) compiler report target triplet and isolate individual
+# words therein to facilitate later processing.
+target.triplet := $(subst -, ,$(shell $(CC) -dumpmachine))
+
+
 #=== operating system ==========================================================
 
 
 # The following systems are defined: Linux, Darwin, Windows. GNU and
 # GNU/kFreeBSD are treated as Linux to get the same options.
 
-uname := $(shell uname)
-
-ifeq ($(findstring $(uname), Linux GNU GNU/kFreeBSD), $(uname))
+ifneq ($(filter linux gnu% kfreebsd, $(target.triplet)),)
   system = Linux
 endif
 
-ifeq ($(uname), Darwin)
+ifneq ($(filter darwin%, $(target.triplet)),)
   system = Darwin
 endif
 
-ifeq ($(filter MINGW% MSYS%, $(uname)), $(uname))
+ifneq ($(filter mingw% cygwin%, $(target.triplet)),)
   system = Windows
 endif
 
-# Unfortunately not all Mingw versions provide a link cc > gcc, therefore
-# gcc is hardcoded here (but not if CC is redefined).
-ifeq ($(system), Windows)
-  ifeq ($(origin CC), default)
-    CC = gcc
-  endif
-endif
-
 # evaluate possible system-specific multiline defines from library makefile
 $(eval $(for$(system)))
 
@@ -472,24 +467,13 @@ $(eval $(for$(system)))
 #=== architecture ==============================================================
 
 
-# native architecture of the build machine
-build.arch := $(shell uname -m)
+# The following CPU names can be processed by pdlibbuilder:
+# i*86    Intel 32 bit
+# x86_64  Intel 64 bit
+# arm     ARM 32 bit
+# aarch64 ARM 64 bit
 
-# Target architecture as reported by compiler. Give precedence to eventual
-# user-defined compiler. The first field of <cpu>-<vendor>-<os> is extracted.
-ifneq ($(origin CXX), default)
-  dumpmachine.cpu = $(firstword $(subst -, ,$(shell $(CXX) -dumpmachine)))
-else
-  dumpmachine.cpu = $(firstword $(subst -, ,$(shell $(CC) -dumpmachine)))
-endif
-
-# Target architecture as reported by compiler is only used for Windows at the
-# moment. For other systems this still has to be tested.
-ifeq ($(system), Windows)
-  target.arch = $(dumpmachine.cpu)
-else
-  target.arch = $(build.arch)
-endif
+target.arch := $(firstword $(target.triplet))
 
 
 ################################################################################
@@ -501,22 +485,27 @@ endif
 
 
 # Set architecture-dependent cflags, mainly for Linux. For Mac and Windows,
-# arch.c.flags are overriden below.
+# arch.c.flags are overriden below. To see gcc's default architecture flags:
+# $ gcc -Q --help=target
 
-# Raspberry Pi 1st generation
-ifeq ($(target.arch), armv6l)
+# ARMv6: Raspberry Pi 1st gen, not detectable from target.arch
+ifeq ($(shell uname), armv6l)
   arch.c.flags = -march=armv6 -mfpu=vfp -mfloat-abi=hard
 
-# Beagle, Udoo, RPi2 etc.
-else ifeq ($(target.arch), armv7l)
+# ARMv7: Beagle, Udoo, RPi2 etc.
+else ifeq ($(target.arch), arm)
   arch.c.flags = -march=armv7-a -mfpu=vfpv3 -mfloat-abi=hard
 
+# ARMv8 64 bit, not tested yet
+else ifeq ($(target.arch), aarch64)
+  arch.c.flags = -mcpu=cortex-a53
+
 # Intel 32 bit, build with SSE and SSE2 instructions
-else ifeq ($(findstring $(target.arch), i386 i686), $(target.arch))
+else ifneq ($(filter i%86, $(target.arch)),)
   arch.c.flags = -march=pentium4 -mfpmath=sse -msse -msse2
 
 # Intel/AMD 64 bit, build with SSE, SSE2 and SSE3 instructions
-else ifeq ($(findstring $(target.arch), x86_64), $(target.arch))
+else ifeq ($(target.arch), x86_64)
   arch.c.flags = -march=core2 -mfpmath=sse -msse -msse2 -msse3
 
 # if none of the above architectures detected