PROJECT_NAME goes into the title, don't scream with ALL CAPS here.
PROJECT_BRIEF also goes into the title. Trailing dots are not used
in English titles and do not look good.

JAVADOC_AUTOBRIEF set to YES because writing "@brief" is excessive
and some members do not use it (resulting in missing short docs).

OPTIMIZE_OUTPUT_FOR_C so that we don't have references to classes
and things like that. We use Doxygen for Themis Core.

TYPEDEF_HIDES_STRUCT set to YES because Themis code style uses
typedefs instead of C tags, so we prefer to not mention them.

INPUT should include the top-level README file and only Themis Core.
USE_MDFILE_AS_MAINPAGE makes sure that README is displayed.
FILE_PATTERNS have to be set explicitly, otherwise Doxygen does not
see our source code.

MACRO_EXPANSION, EXPAND_ONLY_PREDEF, and PREDEFINED are used to
hide pervasive API exporting macros from Doxygen output.

Write these macros in a single line so that Doxygen preprocessor
strips them and does not break function definitions because of that.