This patch adds missing newlines, trims multiple redundant final
newlines into a single one, and trims redundant leading newlines.

According to POSIX, a line is a sequence of zero or more non-' <newline>'
characters plus a terminating '<newline>' character. [1] Files should
normally have at least one final newline character.

C89 [2] and later standards [3] mention a final newline:
"A source file that is not empty shall end in a new-line character,
which shall not be immediately preceded by a backslash character."

Although it is not mandatory for all files to have a final newline
fixed, a more consistent and homogeneous approach brings less of commit
differences issues and a better development experience in certain text
editors and IDEs.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_206
[2] https://port70.net/~nsz/c/c89/c89-draft.html#2.1.1.2
[3] https://port70.net/~nsz/c/c99/n1256.html#5.1.1.2

The bundled library libbcmath is maintained in the php-src and its
build system files are outdated and not used.

* pull-request/2742:
  Fixed bug #66364 (BCMath bcmul ignores scale parameter)

* pull-request/2745:
  Fixed bug #75169 (BCMath errors/warnings bypass error handling)

* PHP-7.2:
  Fixed bug #46781 (BC math handles minus zero incorrectly)

* PHP-7.1:
  Fixed bug #46781 (BC math handles minus zero incorrectly)

* PHP-7.0:
  Fixed bug #46781 (BC math handles minus zero incorrectly)

Actually, there is no negative zero at all.  We obey Postel's law, and
still accept negative zeroes, but we store them as positive zeroes
after the conversion from string, i.e. we normalize before further
processing.

Instead of writing warning messages to `stderr`, we employ PHP's error
handling to raise `E_WARNING` even for the single case where
`bc_rt_error()` has been called, since that did not actually error out.
We choose to call `php_error_docref()` directly in libbcmath, since
there is no upstream, and since other PHP core functionality is already
used in our bundled libbcmath. Accordingly, we remove `rt.c` so it will
not be accidentally used in the future.

Besides adapting a few existing tests, we add new tests so that the
warnings are tested at least once. We also get rid of the Windows
specific tests, since the warning behavior is now supposed to be
platform-agnostic.