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ARROW-18231: [C++][CMake] Add support for overriding optimization level (#15022)

We can overriding optimization level by
`-DCMAKE_CXX_FLAGS_${CONFIG}=-OX` such as
`-DCMAKE_CXX_FLAGS_RELEASE=-O0` with this change.

Note that we can't use CXXFLAGS environment variable because CMake uses
`${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_${CONFIG}}` order and optimization flags are specified
in `CMAKE_CXX_FLAGS_${CONFIG}` not `CMAKE_CXX_FLAGS`. (CMAKE_CXX_FLAGS CMake variable is
initialized by CXXFLAGS environment variable.)

See also: https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_FLAGS.html

> The flags in this variable will be passed to the compiler before
> those in the per-configuration `CMAKE_<LANG>_FLAGS_<CONFIG>` variant,
> and before flags added by the `add_compile_options()` or
> `target_compile_options()` commands.

This change also cleans up the followings:

* C/CXX flags related codes
  * Use `CMAKE_C_FLAGS_${BUILD_TYPE}`/`CMAKE_CXX_FLAGS_${BUILD_TYPE}` instead of putting all flags to `CMAKE_C_FLAGS`/`CMAKE_CXX_FLAGS`
* `externalproject_add()` related codes
  * Extract common options as `EP_XXX`
* Use `string(APPEND)`/`list(APPEND)` as much as possible instead of `set(XXX "${XXX} ...")`/`set(XXX ${XXX} ...)`

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>