Guard Doxygen finder to reduce configure times

11 jobs for master in 12 minutes and 37 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed clang-c++11 #64444
Linux

00:04:01

passed cppcheck #64446
Linux

00:00:30

passed doc #64453
Linux

00:00:20

passed gcc-c++11 #64443
Linux

00:02:45

passed macos-c++11 #64445
Apple

00:03:01

passed VS2013-32-bit #64452
VS2013

00:00:56

passed VS2013-64-bit #64451
VS2013

00:01:01

passed VS2015-32-bit #64450
VS2015

00:01:04

passed VS2015-64-bit #64449
VS2015

00:01:14

passed VS2017-32-bit #64448
VS2017

00:01:12

passed VS2017-64-bit #64447
VS2015

00:01:18