Always expose WITH_GFLAGS option to user (#7990)

Summary:
WITH_GFLAGS option does not work on MSVC.

 I checked the usage of [CMAKE_DEPENDENT_OPTION](https://cmake.org/cmake/help/latest/module/CMakeDependentOption.html). It says if the `depends` condition is not true, it will set the `option` to the value given by `force` and hides the option from the user. Therefore, `CMAKE_DEPENDENT_OPTION(WITH_GFLAGS "build with GFlags" ON "NOT MSVC;NOT MINGW" OFF)` will hide WITH_GFLAGS option from user if it is running on MSVC or MINGW and always set WITH_GFLAGS to be OFF. To expose WITH_GFLAGS option to user, I removed CMAKE_DEPENDENT_OPTION and split the logic into if-else statements

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7990

Reviewed By: jay-zhuang

Differential Revision: D26615755

Pulled By: ajkr

fbshipit-source-id: 33ca39a73423d9516510c15aaf9efb5c4072cdf9

CMakeLists.txt

@@ -88,10 +88,9 @@ if( NOT DEFINED CMAKE_CXX_STANDARD )
 endif()
 
 include(CMakeDependentOption)
-CMAKE_DEPENDENT_OPTION(WITH_GFLAGS "build with GFlags" ON
-  "NOT MSVC;NOT MINGW" OFF)
 
 if(MSVC)
+  option(WITH_GFLAGS "build with GFlags" OFF)
   option(WITH_XPRESS "build with windows built in compression" OFF)
   include(${CMAKE_CURRENT_SOURCE_DIR}/thirdparty.inc)
 else()
@@ -107,6 +106,11 @@ else()
     endif()
   endif()
 
+  if(MINGW)
+    option(WITH_GFLAGS "build with GFlags" OFF)
+  else()
+    option(WITH_GFLAGS "build with GFlags" ON)
+  endif()
   set(GFLAGS_LIB)
   if(WITH_GFLAGS)
     # Config with namespace available since gflags 2.2.2