Add an "--enable-linux" option to ./configure

I keep forgetting that the default option is to build newlib.  This
flag changes the default target to Linux, so I don't keep forgetting
to type "make linux".

Makefile.in

@@ -41,6 +41,8 @@ SHELL := /bin/bash
 PATH := $(INSTALL_DIR)/bin:$(PATH)
 export PATH := $(PATH)
 
+all: @default_target@
+
 newlib: build-gcc-newlib
 
 linux: build-gcc-linux-stage2

configure.ac

@@ -15,6 +15,12 @@ AC_CHECK_LIB(mpc, mpc_add, , [AC_MSG_ERROR(
      [MPC 0.8.1 or greater not found.
      See http://www.multiprecision.org])])
 
+AC_ARG_ENABLE(linux,
+        [Toolchain targets Linux],
+        AC_SUBST(default_target, linux),
+        AC_SUBST(default_target, newlib)
+        )
+
 AC_OUTPUT(
     Makefile
 )