Makefile fix

# "make configure" will autodetect any variables not passed on the
# command line, caching the result in config.mk to be used on any
# subsequent invocations of make.  For example,
#
#   make configure CC=/path/to/my/cc CUDA_DIR=/opt/cuda
#   make
#   make prove
configure :
    @> config.mk
    @echo "CC = $(CC)" | tee -a config.mk
    @echo "FC = $(FC)" | tee -a config.mk
    @echo "NVCC = $(NVCC)" | tee -a config.mk
    @echo "CFLAGS = $(CFLAGS)" | tee -a config.mk
    @echo "CPPFLAGS = $(CPPFLAGS)" | tee -a config.mk
    @echo "FFLAGS = $(FFLAGS)" | tee -a config.mk
    @echo "LDFLAGS = $(LDFLAGS)" | tee -a config.mk
    @echo "LDLIBS = $(LDLIBS)" | tee -a config.mk
    @echo "MAGMA_DIR = $(MAGMA_DIR)" | tee -a config.mk
    @echo "XSMM_DIR = $(XSMM_DIR)" | tee -a config.mk
    @echo "CUDA_DIR = $(CUDA_DIR)" | tee -a config.mk
    @echo "MFEM_DIR = $(MFEM_DIR)" | tee -a config.mk
    @echo "PETSC_DIR = $(PETSC_DIR)" | tee -a config.mk
    @echo "NEK5K_DIR = $(NEK5K_DIR)" | tee -a config.mk
    @echo "Configuration cached in config.mk"

.PHONY : configure