Allow setting GC_INITIAL_HEAP_SIZE for hydra-eval-jobs

This cannot be done in the hydra-evaluator systemd unit, since then
every other Nix process (e.g. hydra-evaluator and nix-prefetch-*) will
also allocate the specified heap size, probably leading to OOM.

src/libhydra/hydra-config.hh

@@ -0,0 +1,52 @@
+#pragma once
+
+#include <map>
+
+#include "util.hh"
+
+struct Config
+{
+    std::map<std::string, std::string> options;
+
+    Config()
+    {
+        using namespace nix;
+
+        /* Read hydra.conf. */
+        auto hydraConfigFile = getEnv("HYDRA_CONFIG");
+        if (pathExists(hydraConfigFile)) {
+
+            for (auto line : tokenizeString<Strings>(readFile(hydraConfigFile), "\n")) {
+                line = trim(string(line, 0, line.find('#')));
+
+                auto eq = line.find('=');
+                if (eq == std::string::npos) continue;
+
+                auto key = trim(std::string(line, 0, eq));
+                auto value = trim(std::string(line, eq + 1));
+
+                if (key == "") continue;
+
+                options[key] = value;
+            }
+        }
+    }
+
+    std::string getStrOption(const std::string & key, const std::string & def = "")
+    {
+        auto i = options.find(key);
+        return i == options.end() ? def : i->second;
+    }
+
+    uint64_t getIntOption(const std::string & key, uint64_t def = 0)
+    {
+        auto i = options.find(key);
+        return i == options.end() ? def : std::stoll(i->second);
+    }
+
+    bool getBoolOption(const std::string & key, bool def = false)
+    {
+        auto i = options.find(key);
+        return i == options.end() ? def : (i->second == "true" || i->second == "1");
+    }
+};