cmake: require CMake v3.10.2

since we dropped the support of xenial, we now have the luxury of using
newer CMake! and by using CMake 3.10.2, we can prevent libfmt from
assuming that we are using C++11, and hence set `CMAKE_CXX_STANDARD` to
11, which will literally append `-std=gnu++11` to `CMAKE_CXX_FLAGS`.
the last `-std` option passed to `g++` takes precendence.
since we've switched over to C++17, and we are using C++17 features.
so, using cmake older than 3.8 breaks the build. because it is CMake 3.8
which stared support `CMAKE_CXX_STANDARD` 17.

- 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("