Remove uses of GNUism get_current_dir_name

This function is part of glibc but is not a POSIX standard function.
Plain old `getcwd` is portable. Fortunately, most of the uses are
copypasta that feed directly into a C++ string construction so this was
easy to replace.

src/algorithms/temp_file.cpp

@@ -95,9 +95,9 @@ std::string get_dir() {
 
     // Get the default temp dir from environment variables.
     if (temp_dir.empty()) {
-        char* cwd = get_current_dir_name();
+        char cwd[512];
+        getcwd(cwd, sizeof(cwd));
         temp_dir = std::string(cwd);
-        free(cwd);
         /*const char* system_temp_dir = nullptr;
         for(const char* var_name : {"TMPDIR", "TMP", "TEMP", "TEMPDIR", "USERPROFILE"}) {
             if (system_temp_dir == nullptr) {

src/algorithms/xp.cpp

@@ -776,19 +776,19 @@ namespace xp {
 
             // Get the default temp dir from environment variables.
             if (temp_dir.empty()) {
-                char* cwd = get_current_dir_name();
+                char cwd[512];
+                getcwd(cwd, sizeof(cwd));
                 temp_dir = std::string(cwd);
-                free(cwd);
                 /*const char *system_temp_dir = nullptr;
                 for (const char *var_name : {"TMPDIR", "TMP", "TEMP", "TEMPDIR", "USERPROFILE"}) {
                     if (system_temp_dir == nullptr) {
                         system_temp_dir = getenv(var_name);
                     }
                 }
                 if (system_temp_dir == nullptr) {
-                    char* cwd = get_current_dir_name();
+                    char cwd[512];
+                    getcwd(cwd, sizeof(cwd));
                     temp_dir = std::string(cwd);
-                    free(cwd);
                 } else {
                     temp_dir = system_temp_dir;
                 }*/

src/subcommand/layout_main.cpp

@@ -135,9 +135,9 @@ int main_layout(int argc, char **argv) {
     if (tmp_base) {
         xp::temp_file::set_dir(args::get(tmp_base));
     } else {
-        char* cwd = get_current_dir_name();
+        char cwd[512];
+        getcwd(cwd, sizeof(cwd));
         xp::temp_file::set_dir(std::string(cwd));
-        free(cwd);
     }
 
     if (!graph.is_optimized()) {

src/subcommand/sort_main.cpp

@@ -217,9 +217,9 @@ int main_sort(int argc, char** argv) {
     if (tmp_base) {
         xp::temp_file::set_dir(args::get(tmp_base));
     } else {
-        char* cwd = get_current_dir_name();
+        char cwd[512];
+        getcwd(cwd, sizeof(cwd));
         xp::temp_file::set_dir(std::string(cwd));
-        free(cwd);
     }
 
     // If required, first of all, optimize the graph so that it is optimized for subsequent algorithms (if required)