On buildbots, the building directory seems to be not a subdir of
a git repo, so a variable expands into an empty string which in turn
results in a syntax error:

--snip--
  -- Looking for in6addr_any - found
  -- Found Git: /data/bowl-builder/arm_cortex-a5/build/sdk/staging_dir/host/bin/git (found version "2.1.4")
  fatal: Not a git repository (or any parent up to mount point /data)
  Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
  CMake Error at CMakeLists.txt:114 (if):
    if given arguments:

      "STREQUAL" "/data/bowl-builder/arm_cortex-a5/build/sdk/build_dir/target-arm_cortex-a5_musl_eabi/libiio-0.11"

    Unknown arguments specified

  -- Configuring incomplete, errors occurred!
--snap--

The patch quotes the if arguments and thus fixes the build.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>