Made the static test part of the makefile more flexible for NaCl.

The following command line now runs the static tests under NaCl. Some crash:

HL_TARGET=x86-64-sse41-nacl \
STATIC_TEST_CXX=~/nacl_sdk/pepper_30/toolchain/linux_x86_newlib/x86_64-nacl/bin/g++ \
STATIC_TEST_LIBS="-Wl,--gc-sections" \
STATIC_TEST_RUNNER="/usr/local/google/home/abadams/nacl_sdk/pepper_30/tools/sel_ldr_x86_64 -B ~/nacl_sdk/pepper_30/tools/irt_core_x86_64.nexe -f" \
make test_static
Merge branch 'master' of github.com:halide/Halide