Allow CUDA runtime library selection (#220)

Makes a change to allow the user to select between the static CUDA
runtime library (default) and the dynamic CUDA runtime library. Does
this by allowing `CUDARTLIB` to be overridden.

src/Makefile

@@ -26,6 +26,7 @@ LIBDIR := $(BUILDDIR)/lib
 OBJDIR := $(BUILDDIR)/obj
 PKGDIR := $(BUILDDIR)/lib/pkgconfig
 ##### target files
+CUDARTLIB  ?= cudart_static
 INCTARGETS := $(INCEXPORTS:%=$(INCDIR)/%)
 LIBSONAME  := $(LIBNAME:%=%.$(NCCL_MAJOR))
 LIBTARGET  := $(LIBNAME:%=%.$(NCCL_MAJOR).$(NCCL_MINOR).$(NCCL_PATCH))
@@ -33,7 +34,7 @@ STATICLIBTARGET := $(STATICLIBNAME)
 PKGTARGET  := $(PKGCONFIGFILE)
 LIBOBJ     := $(LIBSRCFILES:%.cc=$(OBJDIR)/%.o)
 DEPFILES   := $(LIBOBJ:%.o=%.d)
-LDFLAGS    += -L${CUDA_LIB} -lcudart_static -lpthread -lrt -ldl
+LDFLAGS    += -L${CUDA_LIB} -l$(CUDARTLIB) -lpthread -lrt -ldl
 
 DEVICELIB  := $(BUILDDIR)/obj/collectives/device/colldevice.a