doc: fix doxygen build

HTML_TARGET is undefined.  Name the target "doc".

Ensure that we depend on Doxyfile in the binary directory, not the
source directory, as it is a generated file.

Use add_custom_command() to avoid needing to re-run doxygen each time.
We create a "doxygen.txt" file which is used as a dependency stub file.
The presence of this file prevents rebuilds.

src/doc/CMakeLists.txt

@@ -32,14 +32,19 @@
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
 
 find_package(Doxygen)
+
 IF(DOXYGEN_FOUND)
   configure_file(${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile.in ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile @ONLY)
-  FILE(GLOB (DOCUMENTED_FILES ../lib/*.h))
-  add_custom_target(doc ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR} 
-    DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile #  ${DOCUMENTED_FILES}
-    COMMENT "Generating API documentation with Doxygen" VERBATIM)
+  FILE(GLOB (DOCUMENTED_FILES ${CMAKE_PROJECT_DIR}/src/lib/*.h))
+
+  add_custom_command(
+    OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/doxygen.txt
+    COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile >doxygen.txt
+    WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile.in)
+
+  add_custom_target(doc ALL DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/doxygen.txt)
   add_dependencies(doc DOCUMENTED_FILES)
-
-  ADD_CUSTOM_TARGET(${HTML_TARGET} ALL ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile)
+
   INSTALL( DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/html/ DESTINATION share/doc/partio )
 ENDIF(DOXYGEN_FOUND)