Made CMake look for mscgen

Before the path in doxyfile was static and set to
"D:/Programmes/mscgen", which obviously won't work on most systems. Now
CMake tries to find the tool. If this fails, the path is set empty to
let Doxygen look for it inside PATH.

doc/CMakeLists.txt

@@ -12,6 +12,7 @@ if(MACOSX)
  
     set(CMAKE_PROGRAM_PATH ${CMAKE_PROGRAM_PATH} ${ADDITIONAL_PATHS})
 endif()
+
 find_package(Doxygen REQUIRED)
 
 # set the input and output documentation paths
@@ -35,6 +36,11 @@ else()
     set(DOXYGEN_GENERATE_HTMLHELP NO)
 endif()
 
+find_path(MSCGEN_PATH mscgen.exe mscgen)
+if(NOT MSCGEN_PATH)
+    set(MSCGEN_PATH "")
+endif()
+
 # configure the source Doxyfile by copying it and replacing all @variables@
 set(DOXYGEN_CONFIGURED_INPUT ${DOXYGEN_OUTPUT_DIR}/doxyfile)
 configure_file(${DOXYGEN_INPUT_DIR}/doc/doxyfile.in ${DOXYGEN_CONFIGURED_INPUT} @ONLY)

doc/doxyfile.in

@@ -1354,7 +1354,7 @@ CLASS_DIAGRAMS         = YES
 # the mscgen tool resides. If left empty the tool is assumed to be found in the 
 # default search path.
 
-MSCGEN_PATH            = D:/Programmes/mscgen
+MSCGEN_PATH            = "@MSCGEN_PATH@"
 
 # If set to YES, the inheritance and collaboration graphs will hide 
 # inheritance and usage relations if the target is undocumented