generate correct pyconfig.h when cross-compiling

[xdegaye]

BPO	36361
Nosy	@moreati
PRs	bpo-36361: Generate correct pyconfig.h when cross-compiling #12429

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-03-19.09:31:36.070>
labels = ['type-feature', '3.8', 'build']
title = 'generate correct pyconfig.h when cross-compiling'
updated_at = <Date 2019-12-10.08:03:47.869>
user = 'https://github.com/xdegaye'

bugs.python.org fields:

activity = <Date 2019-12-10.08:03:47.869>
actor = 'xdegaye'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Cross-Build']
creation = <Date 2019-03-19.09:31:36.070>
creator = 'xdegaye'
dependencies = []
files = []
hgrepos = []
issue_num = 36361
keywords = ['patch']
message_count = 1.0
messages = ['338335']
nosy_count = 1.0
nosy_names = ['Alex.Willmer']
pr_nums = ['12429']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue36361'
versions = ['Python 3.8']

[xdegaye]

'configure' cache values are set to pessimistic defaults when cross-compiling. One of the most significant, ac_cv_computed_gotos, is set to 'no' in that case. When ac_cv_computed_gotos is set to 'yes' on platforms that support it, it brings a 15-20 % gain in performance. To know if a platform supports computed goto when not cross-compiling, a C test program is built and run using the AC_RUN_IFELSE autoconf macro in configure.ac.

Most platforms provide a way to transfer a file and to run a remote shell, whether it is using ssh or by other means (Android has the 'adb' swiss-knife tool). This is a proposal to extend the AC_RUN_IFELSE macro to also copy the executable locally when cross-compiling. A script uploads it later on the platform, target of the cross-compilation, runs it on this platform, collects its exit status and sets the corresponding cache value accordingly in a config.site file. A second run of 'configure', with the CONFIG_SITE environment variable set to the *absolute* path of this config.site file, allows configuring the build with the correct cache values for this platform.

[kumaraditya303]

Closing as outdated and with the recent efforts, the configure script is now significantly improved with wasm cross compilation.