Squashed commit of the following:

commit 25e7cb77683736a588acb6b30a8ac89e2bd7f56a
Author: Carl Pearson <pearson@illinois.edu>
Date:   Fri Sep 20 13:25:49 2019 -0500

    automatically define PERFECT_HAS_CUDA with nvcc

commit fcc699c165ba515619781aefb378d3c0c4d1093d
Author: Carl Pearson <pearson@illinois.edu>
Date:   Fri Sep 20 13:18:42 2019 -0500

    optional CUDA support

include/perfect/detail/os/linux.hpp

@@ -1,10 +1,14 @@
 #pragma once
 
 #include <cassert>
+#include <cerrno>
 #include <fstream>
 #include <string>
 #include <vector>
 
+#include <cstring>
+#include <iostream>
+
 #include <sched.h>
 #include <sys/types.h>
 #include <unistd.h>
@@ -46,7 +50,14 @@ Result set_governor(const int cpu, const std::string &governor) {
   ofs << governor;
   ofs.close();
   if (ofs.fail()) {
-    return Result::NO_PERMISSION;
+    switch (errno) {
+    case EACCES:
+      return Result::NO_PERMISSION;
+    case ENOENT:
+      return Result::NOT_SUPPORTED;
+    default:
+      return Result::UNKNOWN;
+    }
   }
   return Result::SUCCESS;
 }

include/perfect/init.hpp

@@ -1,6 +1,12 @@
 #pragma once
 
+#ifdef __NVCC__
+#define PERFECT_HAS_CUDA
+#endif
+
+#ifdef PERFECT_HAS_CUDA
 #include <nvml.h>
+#endif
 
 namespace perfect {
 
@@ -11,11 +17,13 @@ Result init() {
   if (init_)
     return Result::SUCCESS;
 
-  // init nvml
+// init nvml
+#ifdef PERFECT_HAS_CUDA
   nvmlReturn_t ret = nvmlInit();
   if (ret != NVML_SUCCESS) {
     return from_nvml(ret);
   }
+#endif
 
   // don't init again if init() called twice
   init_ = true;

include/perfect/result.hpp

@@ -2,22 +2,27 @@
 
 #include <cassert>
 
+#ifdef __NVCC__
+#define PERFECT_HAS_CUDA
+#endif
+
+#ifdef PERFECT_HAS_CUDA
 #include <nvml.h>
+#endif
 
 namespace perfect {
+
 enum class Result {
-  SUCCESS,
-  NVML_NOT_SUPPORTED,
-  NVML_NO_PERMISSION,
-  NVML_UNINITIALIZED,
   NO_PERMISSION,
+  NOT_SUPPORTED,
+  NVML_NO_PERMISSION,
+  NVML_NOT_SUPPORTED,
+  NVML_UNINITIALIZED,
+  SUCCESS,
   UNKNOWN
 };
 
-
-
-
-
+#ifdef PERFECT_HAS_CUDA
 Result from_nvml(nvmlReturn_t nvml) {
   switch (nvml) {
   case NVML_SUCCESS:
@@ -26,6 +31,8 @@ Result from_nvml(nvmlReturn_t nvml) {
     return Result::NVML_UNINITIALIZED;
   case NVML_ERROR_NOT_SUPPORTED:
     return Result::NVML_NOT_SUPPORTED;
+  case NVML_ERROR_NO_PERMISSION:
+    return Result::NVML_NO_PERMISSION;
   case NVML_ERROR_INVALID_ARGUMENT:
   case NVML_ERROR_GPU_IS_LOST:
   case NVML_ERROR_UNKNOWN:
@@ -34,6 +41,7 @@ Result from_nvml(nvmlReturn_t nvml) {
   }
   return Result::UNKNOWN;
 }
+#endif
 
 const char *get_string(const Result &result) {
   switch (result) {
@@ -47,6 +55,8 @@ const char *get_string(const Result &result) {
     return "nvidia-ml returned not supported";
   case Result::NVML_NO_PERMISSION:
     return "nvidia-ml returned no permission";
+  case Result::NOT_SUPPORTED:
+    return "unsupported operation";
   default:
     assert(0 && "unexpected perfect::Result");
   }
@@ -58,7 +68,7 @@ const char *get_string(const Result &result) {
 inline void check(Result result, const char *file, const int line) {
   if (result != Result::SUCCESS) {
     fprintf(stderr, "%s@%d: perfect Error: %s\n", file, line,
-           get_string(result));
+            get_string(result));
     exit(-1);
   }
 }