Even though doxygen is disabled for this build, find_package(Doxygen) fails
if doxygen was present when cmake first checks for it, but it is then removed
before it reaches this part of configure. (install doxygen, make configure,
while that's running after the first mention of doxygen "pkg_delete doxygen"
and let the configure continue; it breaks later with

CMake Warning at /usr/local/share/cmake/Modules/FindDoxygen.cmake:447 (message):
  Unable to determine doxygen version: No such file or directory
Call Stack (most recent call first):
  /usr/local/share/cmake/Modules/FindDoxygen.cmake:610 (_Doxygen_find_doxygen)
  volk/CMakeLists.txt:131 (find_package)

Instead use the results of the previous test here.

Index: volk/CMakeLists.txt
--- volk/CMakeLists.txt.orig
+++ volk/CMakeLists.txt
@@ -128,8 +128,7 @@ endif(ENABLE_ORC)
 ########################################################################
 # Setup doxygen
 ########################################################################
-find_package(Doxygen)
-if(DOXYGEN_FOUND)
+if(ENABLE_DOXYGEN)
     configure_file(
         ${CMAKE_SOURCE_DIR}/Doxyfile.in
         ${CMAKE_BINARY_DIR}/Doxyfile
@@ -140,7 +139,7 @@ if(DOXYGEN_FOUND)
         WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
         COMMENT "Generating documentation with Doxygen" VERBATIM
     )
-endif(DOXYGEN_FOUND)
+endif(ENABLE_DOXYGEN)
 
 ########################################################################
 # Setup the package config file