configure: fix autotools build after gtk-doc removal

Were using a gtk-doc specific file to check if we can
find the srcdir, so configure would fail.
12 jobs for master in 59 minutes (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #298357
gst indent

00:00:47

passed #298356
manifest

00:00:40

 
  Build
passed #298358
build fedora x86_64

00:06:28

passed #298359
build nodebug fedora x86_64

00:06:25

 
  Test
passed #298360
check fedora

00:03:53

passed #298361
integration testsuites fedora

00:12:53

 
  Full Builds
passed #298363
build cerbero android universal

00:17:30

passed #298364
build cerbero cross win32

00:03:50

passed #298365
build cerbero cross win64

00:06:31

passed #298362
build cerbero fedora x86_64

00:04:51

 
  Full Tests
passed #298366
valgrind bad

00:11:41

 
  Apps
passed #298367
android universal examples

00:09:35