Restructure project, add gebs_exists() and gebs_mkdir()

gebs.c

@@ -0,0 +1,16 @@
+#define GEBS_IMPLEMENTATION
+#include "gebs.h"
+
+int main(int argc, char ** argv)
+{
+    gebs_rebuild_self(argc, argv, "cc", "-o", "gebs", __FILE__);
+
+    if (!gebs_exists("build")) {
+        gebs_mkdir("build");
+    }
+
+    if (GEBS_CMD("gcc", "-o", "build/self_rebuild", "example/self_rebuild.c") != 0)
+        return 1;
+
+    return 0;
+}