Use std::filesystem to get current working dir

This doesn't work on GCC 7... TODO: Figure out whether we can safely
bump the requirements to GCC 8.

Signed-off-by: aszlig <aszlig@nix.build>

src/blackhole.cc

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: LGPL-3.0-only
 #include <cstring>
 #include <climits>
+#include <filesystem>
 #include <unistd.h>
 
 #include <sys/stat.h>
@@ -60,15 +61,10 @@ static std::string get_tmpdir(void)
     if (is_writable_dir("/var/tmp"))
         return "/var/tmp";
 
-    int old_errno = errno;
-    char *workdir = get_current_dir_name();
-    errno = old_errno;
-    if (workdir != nullptr) {
-        std::string wdir_str(workdir);
-        free(workdir);
-        if (is_writable_dir(workdir))
-            return workdir;
-    }
+    std::string workdir = std::filesystem::current_path();
+
+    if (is_writable_dir(workdir))
+        return workdir;
 
     LOG(FATAL) << "Unable to get temporary directory.";
     std::abort();

src/rules/parse.cc

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: LGPL-3.0-only
 #include <algorithm>
 #include <iostream>
+#include <filesystem>
 #include <fstream>
 #include <memory>
 #include <sstream>
@@ -322,7 +323,7 @@ std::string make_absolute(const std::string &path)
     if (path.empty() || path[0] == '/')
         return path;
 
-    return std::string(get_current_dir_name()) + '/' + path;
+    return std::filesystem::current_path() / path;
 }
 
 std::optional<Rule> parse_rule_arg(size_t rulepos, const std::string &arg)