Fix Python.h being included after system headers

[krobelus]

My "./dev.sh build" failed with

Compiling kitty/screen.c ...
gcc -MMD -DNDEBUG -DPRIMARY_VERSION=4000 -DSECONDARY_VERSION=37 -DXT_VERSION="0.37.0" -I/home/johannes/git/kitty/dependencies/linux-amd64/include -Wextra -Wfloat-conversion -Wno-missing-field-initializers -Wall -Wstrict-prototypes -std=c11 -pedantic-errors -Werror -O3 -fwrapv -fstack-protector-strong -pipe -fvisibility=hidden -fno-plt -fPIC -D_FORTIFY_SOURCE=2 -flto -fcf-protection=full -march=native -mtune=native -pthread -I/home/johannes/git/kitty/dependencies/linux-amd64/include/libpng16 -I/home/johannes/git/kitty/dependencies/linux-amd64/include -I/home/johannes/git/kitty/dependencies/linux-amd64/include -I/home/johannes/git/kitty/dependencies/linux-amd64/include -I/home/johannes/git/kitty/dependencies/linux-amd64/include/freetype2 -I/home/johannes/git/kitty/dependencies/linux-amd64/include/libpng16 -I/usr/include/uuid -I/home/johannes/git/kitty/dependencies/linux-amd64/include/harfbuzz -I/home/johannes/git/kitty/dependencies/linux-amd64/include/freetype2 -I/home/johannes/git/kitty/dependencies/linux-amd64/include/libpng16 -I/home/johannes/git/kitty/dependencies/linux-amd64/include -I/home/johannes/git/kitty/dependencies/linux-amd64/include/python3.12 -c kitty/screen.c -o build/fast_data_types-kitty-screen.c.o
In file included from /home/johannes/git/kitty/dependencies/linux-amd64/include/python3.12/Python.h:12,
                 from kitty/data-types.h:11,
                 from kitty/text-cache.h:10,
                 from kitty/line.h:10,
                 from kitty/history.h:10,
                 from kitty/lineops.h:9,
                 from kitty/fonts.h:9,
                 from kitty/screen.c:19:
/home/johannes/git/kitty/dependencies/linux-amd64/include/python3.12/pyconfig.h:1875:9: error: "_POSIX_C_SOURCE" redefined
 1875 | #define _POSIX_C_SOURCE 200809L
      |         ^~~~~~~~~~~~~~~
In file included from /usr/include/bits/libc-header-start.h:33,
                 from /usr/include/stdint.h:26,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/include/stdint.h:9,
                 from kitty/charsets.h:9,
                 from kitty/screen.c:17:
/usr/include/features.h:329:10: note: this is the location of the previous definition
  329 | # define _POSIX_C_SOURCE 199506L
      |          ^~~~~~~~~~~~~~~
The following build command failed: /home/johannes/git/kitty/dependencies/linux-amd64/bin/python setup.py develop
exit status 1

Python docs say:

Since Python may define some pre-processor definitions which affect
the standard headers on some systems, you must include Python.h
before any standard headers are included.

Make it so. Since data-types.h is already included first in a lot of
*.c and *.h files, let's use that one.
Also fix the include in launcher.c.

This is similar to commit 8e84b21 (Re-order includes for
"_POSIX_C_SOURCE", 2018-06-02).

Also add an explicit error for this case, to make the resolution more
obvious? Here's the error when I try to include <stdlib.h> in screen.c:

In file included from kitty/screen.c:14:
kitty/data-types.h:11:2: error: #error "Must include \"data-types.h\" before any system headers"
   11 | #error "Must include \"data-types.h\" before any system headers"
      |  ^~~~~
In file included from /home/johannes/git/kitty/dependencies/linux-amd64/include/python3.12/Python.h:12,
                 from kitty/data-types.h:14:
/home/johannes/git/kitty/dependencies/linux-amd64/include/python3.12/pyconfig.h:1875:9: error: "_POSIX_C_SOURCE" redefined
 1875 | #define _POSIX_C_SOURCE 200809L
      |         ^~~~~~~~~~~~~~~
In file included from /usr/include/bits/libc-header-start.h:33,
                 from /usr/include/stdlib.h:26,
                 from kitty/screen.c:13:
/usr/include/features.h:329:10: note: this is the location of the previous definition
  329 | # define _POSIX_C_SOURCE 199506L
      |          ^~~~~~~~~~~~~~~
The following build command failed: /home/johannes/git/kitty/dependencies/linux-amd64/bin/python setup.py develop
exit status 1

[kovidgoyal]

Good idea adding the #error but I think you are missing some re-ordering for the Apple platform since those are failing in CI