add predefined macros for Doxygen

Predefined macros are practices not very well recommended, but can be
better than having no documents at all.  Without those predefined
macros, Doxygen confused for instace PUREFUNC(int foo()) to be a
declaration of PUREFUNC, not foo.

template/Doxyfile.tmpl

@@ -214,12 +214,52 @@ PERLMOD_MAKEVAR_PREFIX =
 # Configuration options related to the preprocessor
 #---------------------------------------------------------------------------
 ENABLE_PREPROCESSING   = YES
-MACRO_EXPANSION        = NO
-EXPAND_ONLY_PREDEF     = NO
+MACRO_EXPANSION        = YES
+EXPAND_ONLY_PREDEF     = YES
 SEARCH_INCLUDES        = YES
 INCLUDE_PATH           = <%=srcdir%> <%=srcdir%>/include
 INCLUDE_FILE_PATTERNS  =
-PREDEFINED             =
+PREDEFINED             = \
+    ALWAYS_INLINE(_)=_ \
+    COLDFUNC= \
+    CONSTFUNC(_)=_ \
+    DEPRECATED(_)=_ \
+    DEPRECATED_BY(__,_)=_ \
+    DEPRECATED_TYPE(__,_)=_ \
+    ERRORFUNC(__,_)=_ \
+    MJIT_FUNC_EXPORTED= \
+    MJIT_STATIC=extern \
+    MJIT_SYMBOL_EXPORT_BEGIN= \
+    MJIT_SYMBOL_EXPORT_END= \
+    NOINLINE(_)=_ \
+    NORETURN(_)=_ \
+    PRINTF_ARGS(_,__,___)=_ \
+    PUREFUNC(_)=_ \
+    RUBY_EXTERN=extern \
+    RUBY_FUNC_EXPORTED= \
+    RUBY_FUNC_NONNULL(__,_)=_ \
+    RUBY_SYMBOL_EXPORT_BEGIN= \
+    RUBY_SYMBOL_EXPORT_END= \
+    WARNINGFUNC(__,_)=_ \
+    _MSC_VER=1924 \
+    __DOXYGEN__ \
+    __GNUC_MINOR__=0 \
+    __GNUC_PATCHLEVEL__=0 \
+    __GNUC__=10 \
+    __STDC_VERSION__=201710L \
+    __clang__=10 \
+    __clang_major__=10 \
+    __clang_minor__=0 \
+    __clang_patchlevel__=0 \
+    __cplusplus=201704L \
+    __has_attribute(_)=0 \
+    __has_builtin(_)=0 \
+    __has_c_attribute(_)=0 \
+    __has_cpp_attribute(_)=0 \
+    __has_declspec_attribute(_)=0 \
+    __has_extension(_)=0 \
+    __has_feature(_)=0 \
+    __has_warning(_)=0
 EXPAND_AS_DEFINED      =
 SKIP_FUNCTION_MACROS   = YES
 #---------------------------------------------------------------------------