Only use custom LTO flags when we actually want LTO

This considerably speeds up linking of release binaries with msvc when testing locally, when LTO is of no concern.

Also properly override b_lto and b_vscrt in all cases for all targets, never leaving them to take default values.

.github/build.sh

@@ -308,7 +308,7 @@ if [[ "$BSH_HOST_PLATFORM-$BSH_HOST_LIBC" == "windows-mingw" ]]; then
 	meson_configure+=$'\t'-Dwindows_icons=false
 fi
 if [[ $BSH_DEBUG_RELEASE-$BSH_STATIC_DYNAMIC == release-static ]]; then
-	meson_configure+=$'\t'-Db_lto=true
+	meson_configure+=$'\t'-Dlto=true
 fi
 if [[ $BSH_HOST_PLATFORM-$BSH_HOST_ARCH == darwin-aarch64 ]]; then
 	meson_configure+=$'\t'--cross-file=.github/macaa64-ghactions.ini

meson.build

@@ -261,16 +261,23 @@ if not is_debug
 	project_cpp_args += args_ccomp_opt
 endif
 
-if not is_debug
-	args_ccomp_lto = []
-	if cpp_compiler.get_id() in [ 'clang', 'clang-cl' ]
-		# use ThinLTO for Clang/LLVM
-		args_ccomp_lto += [ '-flto=thin' ]
-	elif cpp_compiler.get_argument_syntax() == 'msvc'
-		args_ccomp_lto += [ '/GL' ]
+lto = get_option('lto')
+if cpp_compiler.get_argument_syntax() == 'msvc'
+	if lto
+		# apparently meson doesn't know how to tell msvc to use lto: b_lto doesn't even exist when using msvc
+		# https://github.com/mesonbuild/meson/blob/1.4.2/mesonbuild/compilers/mixins/visualstudio.py#L113
+		# so we do it ourselves
+		project_cpp_args += [ '/GL' ]
 		project_link_args += [ '/LTCG' ]
 	endif
-	project_cpp_args += args_ccomp_lto
+else
+	target_options += [ 'b_lto=@0@'.format(lto) ]
+	if lto
+		if cpp_compiler.get_id() in [ 'clang', 'clang-cl' ]
+			# use ThinLTO for Clang/LLVM
+			project_cpp_args += [ '-flto=thin' ]
+		endif
+	endif
 endif
 
 if cpp_compiler.get_argument_syntax() == 'msvc'
@@ -358,10 +365,12 @@ if host_platform == 'windows'
 			endif
 		endforeach
 	endif
-	if tpt_libs_static == 'static' and host_libc == 'msvc'
-		target_options += [
-			'b_vscrt=static_from_buildtype',
-		]
+	if host_libc == 'msvc'
+		if tpt_libs_static == 'static'
+			target_options += [ 'b_vscrt=static_from_buildtype' ]
+		else
+			target_options += [ 'b_vscrt=from_buildtype' ]
+		endif
 	endif
 	foreach def : defs_ccomp_win
 		if cpp_compiler.get_argument_syntax() == 'msvc'

meson_options.txt

@@ -5,6 +5,12 @@ option(
 	value: 'none',
 	description: 'Build statically using libraries present on the system (\'system\') or using prebuilt libraries official builds use (\'prebuilt\')'
 )
+option(
+	'lto',
+	type: 'boolean',
+	value: false,
+	description: 'Link-time optimization, mostly a wrapper around the b_lto built-in option'
+)
 option(
 	'beta',
 	type: 'boolean',