Add experimental support for OpenBLAS library

* Add 'openblas' as mathlib option in configure. Not added to auto-search so
must be specified using --with-openblas

* configure script searches empty tail so that libraries located at default_path_list
roots (ie /usr/local/ + include/openblas_config.h) are found

* Treat ACML as the odd library out in ifdefs since it doesn't conform to typical
BLAS standard. Other libraries like ATLAS should be able to share
OpenBLAS/MKL variants. Add default USE_ACML define in VS projects to match

* Fix 'max' macro define colliding with C++ std::max once openblas headers are included

Usage Notes:

* For best performance, build OpenBLAS with USE_OPENMP=1. When running CNTK, set
OPENBLAS_NUM_THREADS environment var or set numCPUThreads CNTK config variable to the
physical core count or performance will suffer

* OpenBLAS 2.16 (git HEAD) tested in Linux with GCC 4.8.4 and in Windows with
OpenBLAS 2.15 (pre-built binary release + MingGW 64-bit support dlls)

* For Windows, in Math.vcxproj, replace libacml_mp_dll.lib with libopenblas.dll.a and change
USE_ACML define to USE_OPENBLAS. Change ACML_PATH environment variable to your OpenBLAS path.
Modify openblas_config.h as per OpenMathLib/OpenBLAS#708

* On current generation Intel processors, OpenBLAS measures a little faster than
AMD ACML and slower than Intel MKL on MNIST and other examples

Makefile

@@ -132,6 +132,13 @@ ifeq ("$(MATHLIB)","mkl")
   CPPFLAGS += -DUSE_MKL
 endif
 
+ifeq ("$(MATHLIB)","openblas")
+  INCLUDEPATH += $(OPENBLAS_PATH)/include
+  LIBPATH += $(OPENBLAS_PATH)/lib
+  LIBS += -lopenblas -lm -lpthread
+  CPPFLAGS += -DUSE_OPENBLAS
+endif
+
 
 ifdef KALDI_PATH
   ########## Copy includes and defines from $(KALDI_PATH)/src/kaldi.mk ##########