cmake: require CMake v3.10.2

since we dropped the support of xenial, we now have the luxury of using
newer CMake!

- for bionic: https://packages.ubuntu.com/bionic/cmake : 3.10.2
- for CentOS7:
https://dl.fedoraproject.org/pub/epel/7/x86_64/Packages/c/ : 3.13.5

so in this change,

* bump up the required version to v3.10.2
* cleanups

Signed-off-by: Kefu Chai <kchai@redhat.com>

CMakeLists.txt

@@ -1,8 +1,9 @@
-cmake_minimum_required(VERSION 3.5.1)
+cmake_minimum_required(VERSION 3.10.2)
 # remove cmake/modules/FindPython* once 3.12 is required
 
-project(ceph CXX C ASM)
-set(VERSION 15.0.0)
+project(ceph
+  VERSION 15.0.0
+  LANGUAGES CXX C ASM)
 
 if(POLICY CMP0028)
   cmake_policy(SET CMP0028 NEW)

cmake/modules/FindStdFilesystem.cmake

@@ -2,16 +2,8 @@ set(_std_filesystem_test_src
   ${CMAKE_CURRENT_LIST_DIR}/FindStdFilesystem_test.cc)
 
 macro(try_std_filesystem_library _library _result)
-  if(CMAKE_VERSION VERSION_LESS "3.8")
-    # abuse the definition flags, because they are quite
-    # the same as CMAKE_C_FLAGS: they are passed to the
-    # compiler.
-    set(_std_filesystem_try_compile_arg
-      COMPILE_DEFINITIONS "-std=c++17")
-  else()
-    set(_std_filesystem_try_compile_arg
-      CXX_STANDARD 17)
-  endif()
+  set(_std_filesystem_try_compile_arg
+    CXX_STANDARD 17)
   try_compile(_std_filesystem_compiles
     ${CMAKE_CURRENT_BINARY_DIR}
     SOURCES ${_std_filesystem_test_src}

src/CMakeLists.txt

@@ -114,37 +114,13 @@ endif()
 
 
 # require c++17
-if(CMAKE_VERSION VERSION_LESS "3.8")
-  CHECK_CXX_COMPILER_FLAG("-std=c++17" COMPILER_SUPPORTS_CXX17)
-  if(NOT COMPILER_SUPPORTS_CXX17)
-    message(FATAL_ERROR
-      "The compiler ${CMAKE_CXX_COMPILER} has no C++17 support.")
-  endif()
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++17")
-
-  # for compiletest_cxx11_client
-  CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
-  if(NOT COMPILER_SUPPORTS_CXX11)
-    message(FATAL_ERROR
-      "The compiler ${CMAKE_CXX_COMPILER} has no C++11 support.")
-  endif()
-
-  include(CheckCCompilerFlag)
-  CHECK_C_COMPILER_FLAG("-std=gnu99" COMPILER_SUPPORTS_GNU99)
-  if(NOT COMPILER_SUPPORTS_GNU99)
-    message(FATAL_ERROR
-      "The compiler ${CMAKE_C_COMPILER} has no GNU C99 support.")
-  endif()
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=gnu99")
-else()
-  set(CMAKE_CXX_STANDARD 17)
-  set(CMAKE_CXX_EXTENSIONS OFF)
-  set(CMAKE_CXX_STANDARD_REQUIRED ON)
-  set(CMAKE_C_STANDARD 99)
-  # we use `asm()` to inline assembly, so enable the GNU extension
-  set(CMAKE_C_EXTENSIONS ON)
-  set(C_STANDARD_REQUIRED ON)
-endif()
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_EXTENSIONS OFF)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_C_STANDARD 99)
+# we use `asm()` to inline assembly, so enable the GNU extension
+set(CMAKE_C_EXTENSIONS ON)
+set(C_STANDARD_REQUIRED ON)
 
 include(CheckCXXSourceCompiles)
 CHECK_CXX_SOURCE_COMPILES("