[STM32WLx] Only build on compatible STM32 boards

This checks for some system macros to be set, but also includes the
doxygen build to generate documentation.

This reuses the RADIOLIB_EXCLUDE_STM32WLX that the code already checks
for to prevent having to duplicate this macro check in four places.

To detect running inside doxygen, this configures doxygen to predefine
a DOXYGEN macro (it seems no such macros are defined by default by
doxygen).

Doxyfile

@@ -2172,7 +2172,8 @@ INCLUDE_FILE_PATTERNS  =
 # recursively expanded use the := operator instead of the = operator.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-PREDEFINED             = protected=private
+PREDEFINED             = protected=private \
+                         DOXYGEN
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then this
 # tag can be used to specify a list of macro names that should be expanded. The

src/BuildOpt.h

@@ -1010,6 +1010,13 @@
   #define RADIOLIB_STATIC_ARRAY_SIZE   (256)
 #endif
 
+
+// This only compiles on STM32 boards with SUBGHZ module, but also
+// include when generating docs
+#if (!defined(ARDUINO_ARCH_STM32) || !defined(SUBGHZSPI_BASE)) && !defined(DOXYGEN)
+  #define RADIOLIB_EXCLUDE_STM32WLX
+#endif
+
 #if defined(RADIOLIB_DEBUG)
   #if defined(RADIOLIB_BUILD_ARDUINO)
     #define RADIOLIB_DEBUG_PRINT(...) { RADIOLIB_DEBUG_PORT.print(__VA_ARGS__); }