Cleanup gcc version checking code a little.

Also fix this error from cmake 3.5.1:

```
CMake Error at CMakeLists.txt:62 (if):
  if given arguments:

    "CMAKE_COMPILER_IS_GNUCXX" "AND" "CMAKE_CXX_COMPILER_VERSION" "VERSION_LESS_EQUAL" "4.8.5"

  Unknown arguments specified
```

dlib/CMakeLists.txt

@@ -58,11 +58,6 @@ if(has_parent)
    endif()
 endif()
 
-# As of dlib 19.22, GCC 4.8.5 is no longer supported, building will fail, so let users know.
-if (CMAKE_COMPILER_IS_GNUCXX AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS_EQUAL 4.8.5)
-    message(FATAL_ERROR "${CMAKE_CXX_COMPILER} ${CMAKE_CXX_COMPILER_VERSION} is too old for dlib ${VERSION}.\n\
-    Either update your compiler to be fully compliant with C++11 or build an older version of dlib, such as 19.21.")
-endif()
 
 if (COMMAND pybind11_add_module AND MSVC)
    # True when building a python extension module using Visual Studio.  We care

dlib/cmake_utils/set_compiler_specific_options.cmake

@@ -46,8 +46,8 @@ endif()
 
 if(CMAKE_COMPILER_IS_GNUCXX)
    execute_process(COMMAND ${CMAKE_CXX_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION)
-   if (GCC_VERSION VERSION_LESS 4.8)
-      message(FATAL_ERROR "C++11 is required to use dlib, but the version of GCC you are using is too old and doesn't support C++11.  You need GCC 4.8 or newer. ")
+   if (GCC_VERSION VERSION_LESS 4.9)
+      message(FATAL_ERROR "C++11 is required to use dlib, but the version of GCC you are using is too old and doesn't support C++11.  You need GCC 4.9 or newer. ")
    endif()
 endif()