FindCUDA: Add option to set CUDA_HOST_COMPILER via environment

Re-use the `CUDAHOSTCXX` environment variable from the first-class
CUDA language support to specify the host compiler for FindCUDA.

Help/envvar/CUDAHOSTCXX.rst

@@ -7,3 +7,7 @@ determine ``CUDA`` host compiler, after which the value for ``CUDAHOSTCXX`` is
 stored in the cache as :variable:`CMAKE_CUDA_HOST_COMPILER`. For any
 configuration run (including the first), the environment variable will be
 ignored if the :variable:`CMAKE_CUDA_HOST_COMPILER` variable is defined.
+
+This environment variable is primarily meant for use with projects that
+enable ``CUDA`` as a first-class language.  The :module:`FindCUDA`
+module will also use it to initialize its ``CUDA_HOST_COMPILER`` setting.

Modules/FindCUDA.cmake

@@ -105,6 +105,8 @@
 #      the host compiler is constructed with one or more visual studio macros
 #      such as $(VCInstallDir), that expands out to the path when
 #      the command is run from within VS.
+#      If the CUDAHOSTCXX environment variable is set it will
+#      be used as the default.
 #
 #   CUDA_NVCC_FLAGS
 #   CUDA_NVCC_FLAGS_<CONFIG>
@@ -527,7 +529,9 @@ option(CUDA_HOST_COMPILATION_CPP "Generated file extension" ON)
 # Extra user settable flags
 cmake_initialize_per_config_variable(CUDA_NVCC_FLAGS "Semi-colon delimit multiple arguments.")
 
-if(CMAKE_GENERATOR MATCHES "Visual Studio")
+if(DEFINED ENV{CUDAHOSTCXX})
+  set(CUDA_HOST_COMPILER "$ENV{CUDAHOSTCXX}" CACHE FILEPATH "Host side compiler used by NVCC")
+elseif(CMAKE_GENERATOR MATCHES "Visual Studio")
   set(_CUDA_MSVC_HOST_COMPILER "$(VCInstallDir)Tools/MSVC/$(VCToolsVersion)/bin/Host$(Platform)/$(PlatformTarget)")
   if(MSVC_VERSION LESS 1910)
    set(_CUDA_MSVC_HOST_COMPILER "$(VCInstallDir)bin")