[build] Fix build with default configuration for GCC <4.7 (#2160)

CMakeLists.txt

@@ -123,7 +123,11 @@ endif()
 
 # options
 option(CYGWIN_USE_POSIX "Should the POSIX API be used for cygwin. Ignored if the system isn't cygwin." OFF)
-option(ENABLE_CXX11 "Should the c++11 parts (srt-live-transmit) be enabled" ON)
+if (CMAKE_CXX_COMPILER_ID MATCHES "^GNU$" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.7)
+	option(ENABLE_CXX11 "Should the c++11 parts (srt-live-transmit) be enabled" OFF)
+else()
+	option(ENABLE_CXX11 "Should the c++11 parts (srt-live-transmit) be enabled" ON)
+endif()
 option(ENABLE_APPS "Should the Support Applications be Built?" ON)
 option(ENABLE_EXPERIMENTAL_BONDING "Should the EXPERIMENTAL bonding functionality be enabled?" OFF)
 option(ENABLE_TESTING "Should the Developer Test Applications be Built?" OFF)

docs/build/build-options.md

@@ -74,7 +74,7 @@ The `pkg-confg` file (`srt.pc`) will be generated with the `libstdc++` library
 as a dependency. This may be required in some cases where you have an application 
 written in C which therefore won't link against `libstdc++` by default.
 
-**`--enable-c++11`** (default: ON)
+**`--enable-c++11`** (default: ON except for GCC<4.7)
 
 Enable compiling in C++11 mode for those parts that may require it.
 Parts that don't require it will still be compiled in C++03 mode,