lib: introduce struct library

Introduce a new "struct" library which is a port of the Python 3.10 struct
module with a reduced set of API functions. It supports the same format
patterns and conversions while providing the following methods:

    struct = require('struct');

    buf = struct.pack("fmt", args...);
    values = struct.unpack("fmt", buf);

    struct_inst = struct.new("fmt");
    buf = struct_inst.pack(args...);
    values = struct_inst.unpack(buf);

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

CMakeLists.txt

@@ -20,6 +20,7 @@ OPTION(UCI_SUPPORT "UCI plugin support" ON)
 OPTION(RTNL_SUPPORT "Route Netlink plugin support" ON)
 OPTION(NL80211_SUPPORT "Wireless Netlink plugin support" ON)
 OPTION(RESOLV_SUPPORT "NS resolve plugin support" ON)
+OPTION(STRUCT_SUPPORT "Struct plugin support" ON)
 
 OPTION(LEGACY_SUPPORT "Support deprecated syntax features" ON)
 
@@ -151,6 +152,16 @@ IF(RESOLV_SUPPORT)
   ENDIF()
 ENDIF()
 
+IF(STRUCT_SUPPORT)
+  SET(LIBRARIES ${LIBRARIES} struct_lib)
+  ADD_LIBRARY(struct_lib MODULE lib/struct.c)
+  SET_TARGET_PROPERTIES(struct_lib PROPERTIES OUTPUT_NAME struct PREFIX "")
+  CHECK_FUNCTION_EXISTS(frexp FREXP_FUNCTION_EXISTS)
+  IF (NOT FREXP_FUNCTION_EXISTS)
+    TARGET_LINK_LIBRARIES(struct_lib m)
+  ENDIF()
+ENDIF()
+
 IF(UNIT_TESTING)
   ENABLE_TESTING()
   ADD_DEFINITIONS(-DUNIT_TESTING)