cmake_minimum_required() cleanup:

- document version requirements in the top-level file.
- remove incorrect version requirements from sub-dirs.
- set min. required version to 3.5 in top-level file.
- set minimun required version to 3.9 under docs/ .
- make documents building an option (on by default.)
  so, the tree can be built using cmake-3.5 using
 -DBUILD_DOCS=0 on the command line.

src/CMakeLists.txt

@@ -1,5 +1,3 @@
-cmake_minimum_required(VERSION 3.0)
-
 option(ENABLE_64_BIT_WORDS "Set FLAC__BYTES_PER_WORD to 8 (4 is the default)" OFF)
 option(WITH_XMMS "Build XMMS plugin" OFF)
 
@@ -18,7 +16,6 @@ if(WITH_XMMS)
     add_subdirectory("plugin_common")
     add_subdirectory("plugin_xmms")
 endif()
-
 if(BUILD_TESTING)
     add_subdirectory("test_libs_common")
     add_subdirectory("test_libFLAC")

src/libFLAC/CMakeLists.txt

@@ -1,5 +1,3 @@
-cmake_minimum_required(VERSION 3.0)
-
 if(CMAKE_SYSTEM_PROCESSOR MATCHES "[xX]86(_64)?|(AMD|amd)64|i[346]86")
     option(WITH_AVX "Enable AVX, AVX2 optimizations" ON)
 endif()

src/libFLAC/ia32/CMakeLists.txt

@@ -1,5 +1,3 @@
-cmake_minimum_required(VERSION 3.9)
-
 #include_directories("${CMAKE_CURRENT_SOURCE_DIR}")
 add_compile_options(-I${CMAKE_CURRENT_SOURCE_DIR}/)