Support custom docdir

Allow user to specify a custom location for docs. On some Linux distributions
the docs are at /usr/share/doc/pkg-version rather then the unversioned dir.

Signed-off-by: Justin Lecher <[email protected]>

Author: jlec

doc/CMakeLists.txt

@@ -1,4 +1,8 @@
 find_package(LATEX)
+
+if (NOT DOCDIR)
+  set(DOCDIR share/doc/flann)
+endif ()
 
 if (EXISTS ${PDFLATEX_COMPILER} AND EXISTS ${BIBTEX_COMPILER})
     include(${PROJECT_SOURCE_DIR}/cmake/UseLATEX.cmake)
@@ -14,6 +18,6 @@ endif()
 
 install(
     FILES manual.pdf
-    DESTINATION share/doc/flann
+    DESTINATION ${DOCDIR}
     OPTIONAL
 )