Cpp: Remove redundant cmake version checks

In the top-level CMakeLists.txt, a minimum cmake version of 3.15 is
required. Therefore, all checks that were checking whether the cmake
version is >= 3.0 or 3.1 (etc.) are redundant as those will always be
true due to above requirement.

Signed-off-by: Robert Adam <dev@robert-adam.de>

runtime/Cpp/CMakeLists.txt

@@ -36,19 +36,12 @@ endif(WITH_STATIC_CRT)
 
 project(LIBANTLR4)
 
-if(CMAKE_VERSION VERSION_EQUAL "3.0.0" OR
-   CMAKE_VERSION VERSION_GREATER "3.0.0")
-  CMAKE_POLICY(SET CMP0026 NEW)
-  CMAKE_POLICY(SET CMP0054 OLD)
-  CMAKE_POLICY(SET CMP0045 OLD)
-  CMAKE_POLICY(SET CMP0042 OLD)
-endif()
+CMAKE_POLICY(SET CMP0054 OLD)
+CMAKE_POLICY(SET CMP0045 OLD)
+CMAKE_POLICY(SET CMP0042 OLD)
 
-if(CMAKE_VERSION VERSION_EQUAL "3.3.0" OR
-   CMAKE_VERSION VERSION_GREATER "3.3.0")
-  CMAKE_POLICY(SET CMP0059 OLD)
-  CMAKE_POLICY(SET CMP0054 OLD)
-endif()
+CMAKE_POLICY(SET CMP0059 OLD)
+CMAKE_POLICY(SET CMP0054 OLD)
 
 if(APPLE)
   find_library(COREFOUNDATION_LIBRARY CoreFoundation)
@@ -90,21 +83,13 @@ endif()
 # set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DUSE_UTF8_INSTEAD_OF_CODECVT")
 
 # Initialize CXXFLAGS.
-if(CMAKE_VERSION VERSION_GREATER 3.1.0)
-  if(NOT DEFINED CMAKE_CXX_STANDARD)
-    # only set CMAKE_CXX_STANDARD if not already set
-    # this allows the standard to be set by the caller, for example with -DCMAKE_CXX_STANDARD:STRING=17
-    set(CMAKE_CXX_STANDARD 17)
-  endif()
-  set(CMAKE_CXX_STANDARD_REQUIRED ON)
-  set(CMAKE_CXX_EXTENSIONS OFF)
-else()
-  set(CMAKE_CXX_FLAGS                "${CMAKE_CXX_FLAGS} -std=c++17")
-  set(CMAKE_CXX_FLAGS_DEBUG          "${CMAKE_CXX_FLAGS_DEBUG} -std=c++17")
-  set(CMAKE_CXX_FLAGS_MINSIZEREL     "${CMAKE_CXX_FLAGS_MINSIZEREL} -std=c++17")
-  set(CMAKE_CXX_FLAGS_RELEASE        "${CMAKE_CXX_FLAGS_RELEASE} -std=c++17")
-  set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELWITHDEBINFO} -std=c++17")
+if(NOT DEFINED CMAKE_CXX_STANDARD)
+  # only set CMAKE_CXX_STANDARD if not already set
+  # this allows the standard to be set by the caller, for example with -DCMAKE_CXX_STANDARD:STRING=17
+  set(CMAKE_CXX_STANDARD 17)
 endif()
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
 
 set(CMAKE_CXX_FLAGS                  "${CMAKE_CXX_FLAGS} ${MY_CXX_WARNING_FLAGS}")