Add Doxygen generation using CMake

This is a first configuration and probably requires adjustments to be portable.

Doxygen warnings are saved in the ${PUGS_BINARY_DIR}/doxygen.warnings This file will eventually be used to check that documentation is complete.

Closes #12 (closed)

cmake/PugsDoxygen.cmake

+# ---------------------- Doxygen ----------------------
+# check for Doxygen and eventually configure it
+
+# define the option
+option(BUILD_DOXYGEN_DOC "Build Doxygen documentation" ON)
+
+# this is very much inspired by https://vicrucann.github.io/tutorials/quick-cmake-doxygen/
+
+# check if Doxygen is installed
+find_package(Doxygen)
+if (DOXYGEN_FOUND)
+    # set input and output files
+    set(DOXYGEN_IN "${PUGS_SOURCE_DIR}/doc/Doxyfile.in")
+    set(DOXYGEN_OUT "${PUGS_BINARY_DIR}/Doxyfile")
+
+    # request to configure the file
+    configure_file(${DOXYGEN_IN} ${DOXYGEN_OUT} @ONLY)
+    message(STATUS "Configuring Doxygen")
+
+    # note the option ALL which allows to build the docs together with the application
+    add_custom_target(doxygen
+        COMMAND ${DOXYGEN_EXECUTABLE} ${DOXYGEN_OUT}
+        WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+        COMMENT "Generating API documentation with Doxygen"
+        VERBATIM )
+
+    # building doc builds doxygen
+    add_custom_target(doc doxygen)
+else (DOXYGEN_FOUND)
+  message("Doxygen need to be installed to generate the doxygen documentation")
+endif (DOXYGEN_FOUND)