CDRIVER-5876 detect aligned_alloc via CMake instead of feature test macros

[eramongodb]

Resolves CDRIVER-5876. Alternative (preferable?) solution to #1841.

When #1072 was implemented, check_symbol_exists was deliberately avoided due to difficulty with accurately setting the required CMAKE_REQUIRED_* arguments, which are handled independently from the actual definitions and options used with the libraries being built (i.e. the various *_SOURCE macros used to control POSIX, FreeBSD, and Darwin features). However, it seems there are enough platform-specific edge cases concerning the actual availability of aligned_alloc despite efforts to use approprate feature test macros that a build system check is sufficiently well-motivated at this stage.

Therefore, this PR proposes adding a new BSON_HAVE_ALIGNED_ALLOC option to bson-config.h, which is detected using check_symbol_exists alongside other existing checks. This PR also applies a drive-by scan and update to CMAKE_REQUIRED_* variables to ensure better consistency of definitions/options being used for check_*_exists and the built libraries.

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As a reminder, check_symbol_exists behaves as follows:

If the header files define the symbol as a macro it is considered available and assumed to work. If the header files declare the symbol as a function or variable then the symbol must also be available for linking (so intrinsics may not be detected).

and:

The following variables may be set before calling this macro to modify the way the check is run: CMAKE_REQUIRED_*

Although add_definitions, add_compile_options, and other directory-level and target-specific properties are not observed by check_symbol_exists, configuration options such as CMAKE_C_STANDARD and CMAKE_C_FLAGS are observed by check_symbol_exists. It is not entirely clear where/how the line is drawn. For our purposes, maintaining CMAKE_REQUIRED_DEFINITIONS and CMAKE_REQUIRED_LIBRARIES is probably sufficient to ensure consistency between symbol checks and built libraries.

[kevinAlbs]

I like the change to detect the symbol in CMake, but this appears to cause task failures with clang.