Use NO_CUDA_SDK rather than undefine CUDA_HOME

This commit starts a series of small cmake and configuration cleanups
that will improve things when calling non-NVRTC compilers.

The particular cicular change introduced by thi is to allow including
`gpu.h` in NO_SDK mode more cleanly.
Previously we would unset the variable CUDA_HOME but this is hacky
and non intuitive in the light of the `.h.in` configuration file that
will be inserted in a subsequent commit.

This commit replaces the unsetting by setting a new variable NO_CUDA_SDK.

Author: nicolasvasilache

tc/core/CMakeLists.txt

@@ -120,10 +120,10 @@ add_library(
 
   ${TC_CUDA_CROSS_COMPILE_FILES}
 )
-# Unsets CUDA_HOME to compile without SDK
+# Sets NO_CUDA_SDK to compile without SDK
 set_target_properties(tc_core_cuda_no_sdk
                       PROPERTIES COMPILE_FLAGS
-                      "${CMAKE_CXX_FLAGS} -UCUDA_HOME -Wl,--no-undefined")
+                      "${CMAKE_CXX_FLAGS} -DNO_CUDA_SDK")
 target_include_directories(tc_core_cuda_no_sdk PUBLIC ${LLVM_INCLUDE_DIRS})
 target_link_libraries(
   tc_core_cuda_no_sdk

tc/core/gpu.h

@@ -17,7 +17,7 @@
 
 // Conditionally include CUDA-specific headers.  This file should compile even
 // without them.
-#ifdef CUDA_HOME
+#if defined(CUDA_HOME)
 #include "tc/core/cuda/cuda.h"
 #endif
 
@@ -27,7 +27,7 @@ namespace tc {
 /// The call is forwarded to the appropriate GPU driver (CUDA in particular).
 /// If a thread has no associated GPU device, return 0.
 inline size_t querySharedMemorySize() {
-#ifdef CUDA_HOME
+#if defined(CUDA_HOME) && !defined(NO_CUDA_SDK)
   return CudaGPUInfo::GPUInfo().SharedMemorySize();
 #else
   return 0;