util: Add function os_unset_option/os_set_option for latter use

It's will be used to replace SetEnvironmentVariableA,putenv on windows
and putenv,setenv on non-windows

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Antonio Ospite <antonio.ospite@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38640>

src/util/os_misc.c

@@ -328,6 +328,25 @@ exit_mutex:
    return opt;
 }
 
+void
+os_set_option(const char *name, const char *value, bool override)
+{
+   if (override == false) {
+      if (os_get_option(name)) {
+         return;
+      }
+   }
+#if DETECT_OS_WINDOWS
+   SetEnvironmentVariableA(name, value);
+#else
+   if (value == NULL) {
+      unsetenv(name);
+   } else {
+      setenv(name, value, 1);
+   }
+#endif
+}
+
 /**
  * Return the size of the total physical memory.
  * \param size returns the size of the total physical memory

src/util/os_misc.h

@@ -128,6 +128,26 @@ os_get_option_secure_dup(const char *name);
 const char *
 os_get_option_cached(const char *name);
 
+/*
+ * Set an option(environment variable or os property) in a platform independent way.
+ * If @overwrite is true:
+ *   On windows, equivalent to SetEnvironmentVariableA.
+ *   On non-windows, when @value is NULL, equivalent to unsetenv, otherwise setenv(override=1).
+ * If @overwrite is false:
+ *   When name already exist, do nothing, otherwise, equivalent to @overwrite is true
+ */
+void
+os_set_option(const char *name, const char *value, bool override);
+
+/**
+ * Unset an option through os_set_option
+ */
+static inline void
+os_unset_option(const char *name)
+{
+   os_set_option(name, NULL, true);
+}
+
 /*
  * Get the total amount of physical memory available on the system.
  */