Also use formerly gcc-only flags with clang:

https://github.com/pothosware/SoapyRTLSDR/commit/448c9d0d326c2600905b7ce84222ff9d72ba2189
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -1,7 +1,7 @@
 ########################################################################
 # Build Soapy SDR support module for RTL-SDR Devices
 ########################################################################
-cmake_minimum_required(VERSION 2.8.7)
+cmake_minimum_required(VERSION 2.8.12)
 project(SoapyRTLSDR CXX)
 
 find_package(SoapySDR "0.4.0" NO_MODULE REQUIRED)
@@ -22,7 +22,7 @@
 include_directories(${RTLSDR_INCLUDE_DIRS})
 
 #enable c++11 features
-if(CMAKE_COMPILER_IS_GNUCXX)
+if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU" OR "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
 
     #C++11 is a required language feature for this project
     include(CheckCXXCompilerFlag)
@@ -37,9 +37,8 @@
     list(APPEND RTLSDR_LIBRARIES -pthread)
 
     #disable warnings for unused parameters
-    add_definitions(-Wno-unused-parameter)
-
-endif(CMAKE_COMPILER_IS_GNUCXX)
+    add_compile_options(-Wall -Wextra -Wno-unused-parameter)
+endif()
 
 if (APPLE)
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wc++11-extensions")