Split setenv/unsetenv by platform

Windows apparently doesn't have unsetenv() at all, so use the WinAPI
native envar functions instead of the usual C ones.

c/builtins.c

@@ -391,21 +391,17 @@ static value_t fl_os_getenv(value_t *args, uint32_t nargs)
 
 static value_t fl_os_setenv(value_t *args, uint32_t nargs)
 {
-    char *name;
-    char *val;
-    int result;
+    const char *name;
+    const char *value;
 
     argcount("os.setenv", nargs, 2);
     name = tostring(args[0], "os.setenv");
     if (args[1] == FL_F) {
-        unsetenv(name);
+        value = 0;
     } else {
-        val = tostring(args[1], "os.setenv");
-        result = setenv(name, val, 1);
-        if (result != 0) {
-            lerror(ArgError, "os.setenv: invalid environment variable");
-        }
+        value = tostring(args[1], "os.setenv");
     }
+    os_setenv(name, value);
     return FL_T;
 }
 

c/os.h

@@ -3,3 +3,4 @@ void get_cwd(char *buf, size_t size);
 int set_cwd(char *buf);
 char *get_exename(char *buf, size_t size);
 int os_path_exists(const char *path);
+void os_setenv(const char *name, const char *value);

c/os_unix.c

@@ -113,3 +113,16 @@ int os_path_exists(const char *path)
     }
     return FL_T;
 }
+
+void os_setenv(const char *name, const char *value)
+{
+    if (value) {
+        if (setenv(name, value, 1) != 0) {
+            lerror(ArgError, "os.setenv: cannot set environment variable");
+        }
+    } else {
+        if (unsetenv(name) != 0) {
+            lerror(ArgError, "os.setenv: cannot unset environment variable");
+        }
+    }
+}

c/os_windows.c

@@ -32,6 +32,8 @@
 #include "flisp.h"
 
 #include "error.h"
+
+#include "argcount.h"
 #include "os.h"
 
 void path_to_dirname(char *path)
@@ -73,3 +75,14 @@ int os_path_exists(const char *path)
     }
     return FL_T;
 }
+
+void os_setenv(const char *name, const char *value)
+{
+    if (!SetEnvironmentVariable(name, value)) {
+        if (value) {
+            lerror(ArgError, "os.setenv: cannot set environment variable");
+        } else {
+            lerror(ArgError, "os.setenv: cannot unset environment variable");
+        }
+    }
+}