Use DOI as link

The bibliographic styles available to Doxygen do not process the
DOI field of BibTeX files. The DOI field is now used to create the
URL field programmatically.

doc/doxygen/CMakeLists.txt

@@ -7,13 +7,19 @@ if(DOXYGEN_FOUND)
     DEPENDS EspressoConfig
   )
 
+  add_custom_command(
+    OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/bibliography.bib
+    COMMAND sed -r
+     "'s/^ *doi *= *([^0-9]+)(10\\.[0-9]+)/url=\\1https:\\/\\/dx.doi.org\\/\\2/'"
+     ${CMAKE_CURRENT_SOURCE_DIR}/bibliography.bib >
+     ${CMAKE_CURRENT_BINARY_DIR}/bibliography.bib
+    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/bibliography.bib
+  )
+
   set(DOXYGEN_IN ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile.in)
   set(DOXYGEN_OUT ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile)
 
   configure_file(${DOXYGEN_IN} ${DOXYGEN_OUT} @ONLY)
-  configure_file(${CMAKE_CURRENT_SOURCE_DIR}/bibliography.bib
-                 ${CMAKE_CURRENT_BINARY_DIR}/bibliography.bib
-                 COPYONLY)
 
   add_custom_target(doxygen
     COMMAND ${DOXYGEN_EXECUTABLE} ${DOXYGEN_OUT}