some progress on dos vesa, but not quite

Common/def.h

@@ -21,19 +21,25 @@
 #define COM_DEF_TARGET_LINUX 1
 #define COM_DEF_TARGET_WASM 2
 #define COM_DEF_TARGET_COSMO 3
+#define COM_DEF_TARGET_DOS 4
 
+#ifndef COM_DEF_TARGET
 #if defined(__wasm__)
 #define COM_DEF_TARGET COM_DEF_TARGET_WASM
-#define com_def_export_sym(m_s) __attribute__((export_name(m_s)))
 #elif defined(__linux__)
 #define COM_DEF_TARGET COM_DEF_TARGET_LINUX
-#define com_def_export_sym(m_s) __attribute__((visibility("default")))
 #elif defined(__COSMOPOLITAN__)
 #define COM_DEF_TARGET COM_DEF_TARGET_COSMO
-#define com_def_export_sym(m_s) __attribute__((visibility("default")))
 #else
 #error Unknown target.
 #endif
+#endif
+
+#if COM_DEF_TARGET == COM_DEF_TARGET_WASM
+#define com_def_export_sym(m_s) __attribute__((export_name(m_s)))
+#else
+#define com_def_export_sym(m_s) __attribute__((visibility("default")))
+#endif
 
 #define COM_DEF_CPU_AMD64 1
 #define COM_DEF_CPU_X86 2