Fix minor CMake compatibility issues

Author: zuckschwerdt

Diff for: CMakeLists.txt

@@ -1,7 +1,7 @@
 ########################################################################
 # Project setup
 ########################################################################
-cmake_minimum_required(VERSION 2.8.9)
+cmake_minimum_required(VERSION 2.8.12)
 project(SoapyPlutoSDR CXX)
 
 # select build type to get optimization flags

Diff for: FindLibAD9361.cmake

@@ -25,7 +25,9 @@ find_library(LibAD9361_LIBRARY NAMES ad9361 libad9361
 set(LibAD9361_VERSION ${PC_LibAD9361_VERSION})
 
 include(FindPackageHandleStandardArgs)
+# Note that `FOUND_VAR LibIIO_FOUND` is needed for cmake 3.2 and older.
 find_package_handle_standard_args(LibAD9361
+                                  FOUND_VAR LibAD9361_FOUND
                                   REQUIRED_VARS LibAD9361_LIBRARY LibAD9361_INCLUDE_DIR
                                   VERSION_VAR LibAD9361_VERSION)
 

Diff for: FindLibIIO.cmake

@@ -25,7 +25,9 @@ find_library(LibIIO_LIBRARY NAMES iio libiio
 set(LibIIO_VERSION ${PC_LibIIO_VERSION})
 
 include(FindPackageHandleStandardArgs)
+# Note that `FOUND_VAR LibIIO_FOUND` is needed for cmake 3.2 and older.
 find_package_handle_standard_args(LibIIO
+                                  FOUND_VAR LibIIO_FOUND
                                   REQUIRED_VARS LibIIO_LIBRARY LibIIO_INCLUDE_DIR
                                   VERSION_VAR LibIIO_VERSION)
 

Diff for: FindLibUSB.cmake

@@ -44,7 +44,9 @@ set(LibUSB_VERSION ${PC_LibUSB_VERSION})
 include(FindPackageHandleStandardArgs)
 # handle the QUIETLY and REQUIRED arguments and set LibUSB_FOUND to TRUE
 # if all listed variables are TRUE
+# Note that `FOUND_VAR LibIIO_FOUND` is needed for cmake 3.2 and older.
 find_package_handle_standard_args(LibUSB
+                                  FOUND_VAR LibUSB_FOUND
                                   REQUIRED_VARS LibUSB_LIBRARY LibUSB_INCLUDE_DIR
                                   VERSION_VAR LibUSB_VERSION)