vim-patch:8.2.0665: wrongly assuming Python executable is called "python"

Problem:    Wrongly assuming Python executable is called "python".
Solution:   Use detected python command. (Ken Takata, closes vim/vim#6016)
            Also use CheckFunction if possible.
a161cb5ddd

CheckPython() is used only in test_terminal.vim as of patch 8.2.1794.
test_terminal.vim is no-opt in Neovim.
I ported it only for merging Vim patches.

N/A patches for version.c:

vim-patch:8.1.2239: CI fails when running tests without building Vim

Problem:    CI fails when running tests without building Vim.
Solution:   Skip creating doc tags if the execute does not exist.
1a577433ac

vim-patch:8.1.2242: creating docs tags uses user preferences

Problem:    Creating docs tags uses user preferences. (Tony Mechelynck)
Solution:   Add "--clean".
70def98a95

vim-patch:8.1.2247: "make vimtags" does not work in runtime/doc

Problem:    "make vimtags" does not work in runtime/doc.
Solution:   Test existence with "which" instead of "test -x". (Ken Takata)
e890b9f5dd

vim-patch:8.1.2249: "make vimtags" does not print any message

Problem:    "make vimtags" does not print any message.
Solution:   Add a message that the tags have been updated.
d047840ce4

vim-patch:8.1.2253: using "which" to check for an executable is not reliable

Problem:    Using "which" to check for an executable is not reliable.
Solution:   Use "command -v" instead.  Also exit with error code when
            generating tags has an error. (closes vim/vim#5174)
ad4de52510

vim-patch:8.2.0354: Python 3.9 does not define _Py_DEC_REFTOTAL

Problem:    Python 3.9 does not define _Py_DEC_REFTOTAL. (Zdenek Dohnal)
Solution:   Remove it, it was only for debugging.
a65bb53514

vim-patch:8.2.0603: configure does not detect moonjit

Problem:    Configure does not detect moonjit.
Solution:   Add check for moonjit. (Shlomi Fish, closes vim/vim#5947)
f49e564082

vim-patch:8.2.0609: configure does not detect moonjit correctly

Problem:    Configure does not detect moonjit correctly.
Solution:   Double the brackets. (Ozaki Kiichi)
ad4dc83389

vim-patch:8.2.1570: configure check for dirfd() does not work on HPUX

Problem:    Configure check for dirfd() does not work on HPUX. (Michael Osipov)
Solution:   Use AC_TRY_LINK instead of AC_TRY_COMPILE. (closes vim/vim#6838)
9d8bfae50f

vim-patch:8.2.1790: MS-Windows with Python: crash when executed from Vifm

Problem:    MS-Windows with Python: crash when executed from Vifm.
Solution:   Use NUL instead of CONIN. (Ken Takata, closes vim/vim#7061, closes vim/vim#7053)
794771cfd8

vim-patch:8.2.1792: Configure does not recognize Racket 6.1+

Problem:    Configure does not recognize Racket 6.1+.
Solution:   Add a check for "rktio". (closes vim/vim#7062)
588d241d44

src/nvim/testdir/check.vim

@@ -25,6 +25,14 @@ func CheckFunction(name)
   endif
 endfunc
 
+" Command to check for the presence of python.  Argument should have been
+" obtained with PythonProg()
+func CheckPython(name)
+  if a:name == ''
+    throw 'Skipped: python command not available'
+  endif
+endfunc
+
 " Command to check for running on MS-Windows
 command CheckMSWindows call CheckMSWindows()
 func CheckMSWindows()