Doxygen: create tag file.

Turned off INLINE_SIMPLE_STRUCTS, as it causes bugs in the tagfile.

Doxyfile

@@ -319,7 +319,8 @@ INLINE_GROUPED_CLASSES = NO
 # structs, classes, and unions are shown on a separate page (for HTML and Man
 # pages) or section (for LaTeX and RTF).
 
-INLINE_SIMPLE_STRUCTS  = YES
+# `YES` causes unclosed <compound> element for TgaImporter class in tag file
+INLINE_SIMPLE_STRUCTS  = NO
 
 # When TYPEDEF_HIDES_STRUCT is enabled, a typedef of a struct, union, or enum
 # is documented as struct, union, or enum with the name of the typedef. So
@@ -1569,7 +1570,7 @@ TAGFILES               =
 # When a file name is specified after GENERATE_TAGFILE, doxygen will create
 # a tag file that is based on the input files it reads.
 
-GENERATE_TAGFILE       =
+GENERATE_TAGFILE       = build/doc/magnum.tag
 
 # If the ALLEXTERNALS tag is set to YES all external classes will be listed
 # in the class index. If set to NO only the inherited external classes