auto-select make based on current architecture

The best way to make it auto-select an architecture (without duplicating
the compile code) is to have it call a recursive make. I don't know of a
way to have make 'jump' to a target based on a conditional without doing
it this way.

Makefile

@@ -1,8 +1,20 @@
 CC	= clang
 CFLAGS	= -std=gnu99 -ggdb
+UNAME_M := $(shell uname -m)
+
+.PHONY: x86 x86_64 arm
 
 all:
-	$(error Please choose an architecture to build for: "make arm", "make x86", "make x86_64")
+ifeq ($(UNAME_M),x86_64)
+	$(MAKE) x86_64
+endif
+ifeq ($(UNAME_M),x86)
+	$(MAKE) x64
+endif
+ifneq (,$(findstring arm,$(UNAME_M)))
+	$(MAKE) arm
+endif
+
 
 arm: sample-target sample-library.so
 	$(CC) -marm $(CFLAGS) -DARM -o inject utils.c ptrace.c inject-arm.c -ldl

README.md

@@ -21,17 +21,19 @@
 
 ## Compiling
 
-* arm:
+* Simply running `make` should automatically select and build for the correct architecture, but if this fails (or you would like to select the target manually), run one of the following make commands:
 
-        make arm
+    * arm:
 
-* x86:
+            make arm
 
-        make x86
+    * x86:
 
-* x86_64:
+            make x86
 
-        make x86_64
+    * x86_64:
+
+            make x86_64
 
 ## Usage