Drop c_std=gnu99 to avoid Meson deprecation warning

Meson >= 1.3.0 complains about "c_std=gnu99" when building with MSVC:

    DEPRECATION: None of the values ['gnu99'] are supported by the c compiler.
    However, the deprecated gnu99 std currently falls back to c99.
    This will be an error in the future.
    If the project supports both GNU and MSVC compilers, a value such as
    "c_std=gnu11,c11" specifies that GNU is prefered but it can safely fallback to plain c11.

This breaks CI for projects that embed proxy-libintl as a subproject and
build with --fatal-meson-warnings.

We can't trivially fix this with "c_std=gnu99,c99", since that syntax
isn't supported by older Meson versions and we probably don't want to
require Meson 1.3.0+.  Instead, switch to plain C99, and define
_POSIX_C_SOURCE so we still have access to the needed function prototypes.

Fix Windows static build

On Windows, static compilation needs G_INTL_STATIC_COMPILATION
to be also defined during compilation. Else functions are exported with
__declspec(dllexport) which causes issues when importing symbols during
linkage with the static library.

Set G_INTL_STATIC_COMPILATION when making a static library (#2)

Include a leaner windows.h which avoids a lot of useless stuff