Add sdl2 to BUILDING.md description

[andy31415]

Describe what to do to get -with-ui build support for linux and mac.

[github-actions]

PR #26132: Size comparison from 2b3d5c3 to 0380874

Increases (1 build for cc32xx)

platform	target	config	section	2b3d5c3	0380874	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	20328411	20328412	1	0.0

Full report (1 build for cc32xx)

platform	target	config	section	2b3d5c3	0380874	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	643249	643249	0	0.0
			(read/write)	203848	203848	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197248	197248	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	933224	933224	0	0.0
			.debug_aranges	87792	87792	0	0.0
			.debug_frame	302140	302140	0	0.0
			.debug_info	20328411	20328412	1	0.0
			.debug_line	2687890	2687890	0	0.0
			.debug_loc	2838930	2838930	0	0.0
			.debug_ranges	288072	288072	0	0.0
			.debug_str	3042155	3042155	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	104401	104401	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	377963	377963	0	0.0
			.symtab	256976	256976	0	0.0
			.text	536728	536728	0	0.0