Improve configure script so ninja can be run without a wrapper

[ksperling-apple]

This is done by using gn --script-executable to make Python scripts / actions use the venv without having to activate it first. Tweak zap_execution.py to look for zap-cli in the venv bin directory in preference to PATH so this works for code generation.

Plus some minor improvements:

• Remove special casing of the build environment directory for in-tree builds and be more helpful when running configure without arguments.
• Handle VAR=VALUE arguments as environment variable assignments
• Guess CXX based on whether cc appears to be gcc or clang when CC and CXX are not provided.

[github-actions]

PR #26379: Size comparison from 9556cc3 to 38575f2

Increases (1 build for cc32xx)

platform	target	config	section	9556cc3	38575f2	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	19603114	19603115	1	0.0

Full report (1 build for cc32xx)

platform	target	config	section	9556cc3	38575f2	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	604866	604866	0	0.0
			(read/write)	204156	204156	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197568	197568	0	0.0
			.comment	206	206	0	0.0
			.data	1468	1468	0	0.0
			.debug_abbrev	957098	957098	0	0.0
			.debug_aranges	101104	101104	0	0.0
			.debug_frame	341416	341416	0	0.0
			.debug_info	19603114	19603115	1	0.0
			.debug_line	2666235	2666235	0	0.0
			.debug_line_str	513	513	0	0.0
			.debug_loc	33340	33340	0	0.0
			.debug_loclists	1488792	1488792	0	0.0
			.debug_ranges	4984	4984	0	0.0
			.debug_rnglists	94291	94291	0	0.0
			.debug_str	3108084	3108084	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	104346	104346	0	0.0
			.shstrtab	265	265	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	482872	482872	0	0.0
			.symtab	287120	287120	0	0.0
			.text	498396	498396	0	0.0

[github-actions]

PR #26379: Size comparison from c984f0b to b8edd5c

Decreases (1 build for cc32xx)

platform	target	config	section	c984f0b	b8edd5c	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	19609757	19609756	-1	-0.0

Full report (1 build for cc32xx)

platform	target	config	section	c984f0b	b8edd5c	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	605090	605090	0	0.0
			(read/write)	204164	204164	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197576	197576	0	0.0
			.comment	206	206	0	0.0
			.data	1468	1468	0	0.0
			.debug_abbrev	957621	957621	0	0.0
			.debug_aranges	101136	101136	0	0.0
			.debug_frame	341512	341512	0	0.0
			.debug_info	19609757	19609756	-1	-0.0
			.debug_line	2666632	2666632	0	0.0
			.debug_line_str	513	513	0	0.0
			.debug_loc	33340	33340	0	0.0
			.debug_loclists	1489225	1489225	0	0.0
			.debug_ranges	4984	4984	0	0.0
			.debug_rnglists	94315	94315	0	0.0
			.debug_str	3108982	3108982	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	104354	104354	0	0.0
			.shstrtab	265	265	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	483384	483384	0	0.0
			.symtab	287328	287328	0	0.0
			.text	498612	498612	0	0.0