v.builder: fix `-cc msvc -cflags /O2` and support CFLAGS and LDFLAGS env vars

vlib/v/builder/c.v

@@ -29,9 +29,6 @@ pub fn (mut b Builder) build_c(v_files []string, out_file string) {
 }
 
 pub fn (mut b Builder) compile_c() {
-	if os.user_os() != 'windows' && b.pref.ccompiler == 'msvc' && !b.pref.out_name.ends_with('.c') {
-		verror('Cannot build with msvc on $os.user_os()')
-	}
 	// cgen.genln('// Generated by V')
 	// println('compile2()')
 	if b.pref.is_verbose {

vlib/v/builder/cc.v

@@ -517,7 +517,7 @@ fn (mut v Builder) cc() {
 		return
 	}
 	// Cross compiling for Windows
-	if v.pref.os == .windows {
+	if v.pref.os == .windows && v.pref.ccompiler != 'msvc' {
 		$if !windows {
 			v.cc_windows_cross()
 			return
@@ -618,11 +618,9 @@ fn (mut v Builder) cc() {
 			v.ccoptions.post_args << libs
 		}
 		//
-		$if windows {
-			if ccompiler == 'msvc' {
-				v.cc_msvc()
-				return
-			}
+		if ccompiler == 'msvc' {
+			v.cc_msvc()
+			return
 		}
 		//
 		all_args := v.all_args(v.ccoptions)

vlib/v/builder/msvc.v

@@ -268,17 +268,23 @@ pub fn (mut v Builder) cc_msvc() {
 	r := v.cached_msvc
 	if r.valid == false {
 		verror('Cannot find MSVC on this OS')
-		return
 	}
 	out_name_obj := os.real_path(v.out_name_c + '.obj')
 	out_name_pdb := os.real_path(v.out_name_c + '.pdb')
 	out_name_cmd_line := os.real_path(v.out_name_c + '.rsp')
+	//
+	env_cflags := os.getenv('CFLAGS')
+	env_ldflags := os.getenv('LDFLAGS')
+	mut a := [v.pref.cflags]
+	if env_cflags != '' {
+		a << env_cflags
+	}
 	// Default arguments
 	// volatile:ms enables atomic volatile (gcc _Atomic)
 	// -w: no warnings
 	// 2 unicode defines
 	// /Fo sets the object file name - needed so we can clean up after ourselves properly
-	mut a := ['-w', '/we4013', '/volatile:ms', '/Fo"$out_name_obj"']
+	a << ['-w', '/we4013', '/volatile:ms', '/Fo"$out_name_obj"']
 	if v.pref.is_prod {
 		a << '/O2'
 		a << '/MD'
@@ -323,7 +329,7 @@ pub fn (mut v Builder) cc_msvc() {
 		*/
 	}
 	if v.pref.sanitize {
-		println('Sanitize not supported on msvc.')
+		eprintln('Sanitize not supported on msvc.')
 	}
 	// The C file we are compiling
 	// a << '"$TmpPath/$v.out_name_c"'
@@ -332,7 +338,6 @@ pub fn (mut v Builder) cc_msvc() {
 	// Not all of these are needed (but the compiler should discard them if they are not used)
 	// these are the defaults used by msbuild and visual studio
 	mut real_libs := ['kernel32.lib', 'user32.lib', 'advapi32.lib']
-	// sflags := v.get_os_cflags().msvc_string_flags()
 	sflags := msvc_string_flags(v.get_os_cflags())
 	real_libs << sflags.real_libs
 	inc_paths := sflags.inc_paths
@@ -362,6 +367,9 @@ pub fn (mut v Builder) cc_msvc() {
 		a << '/OPT:ICF'
 	}
 	a << lib_paths
+	if env_ldflags != '' {
+		a << env_ldflags
+	}
 	args := a.join(' ')
 	// write args to a file so that we dont smash createprocess
 	os.write_file(out_name_cmd_line, args) or {
@@ -371,12 +379,14 @@ pub fn (mut v Builder) cc_msvc() {
 	// It is hard to see it at first, but the quotes above ARE balanced :-| ...
 	// Also the double quotes at the start ARE needed.
 	v.show_cc(cmd, out_name_cmd_line, args)
+	if os.user_os() != 'windows' && !v.pref.out_name.ends_with('.c') {
+		verror('Cannot build with msvc on $os.user_os()')
+	}
 	util.timing_start('C msvc')
 	res := os.execute(cmd)
 	if res.exit_code != 0 {
 		eprintln(res.output)
 		verror('msvc error')
-		return
 	}
 	util.timing_measure('C msvc')
 	if v.pref.show_c_output {
@@ -394,7 +404,6 @@ fn (mut v Builder) build_thirdparty_obj_file_with_msvc(path string, moduleflags
 	msvc := v.cached_msvc
 	if msvc.valid == false {
 		verror('Cannot find MSVC on this OS')
-		return
 	}
 	// msvc expects .obj not .o
 	mut obj_path := '${path}bj'
@@ -410,7 +419,12 @@ fn (mut v Builder) build_thirdparty_obj_file_with_msvc(path string, moduleflags
 	defines := flags.defines.join(' ')
 	include_string := '-I "$msvc.ucrt_include_path" -I "$msvc.vs_include_path" -I "$msvc.um_include_path" -I "$msvc.shared_include_path" $inc_dirs'
 	// println('cfiles: $cfiles')
-	mut oargs := []string{}
+	env_cflags := os.getenv('CFLAGS')
+	env_ldflags := os.getenv('LDFLAGS')
+	mut oargs := [v.pref.cflags]
+	if env_cflags != '' {
+		oargs << env_cflags
+	}
 	if v.pref.is_prod {
 		oargs << '/O2'
 		oargs << '/MD'
@@ -419,6 +433,9 @@ fn (mut v Builder) build_thirdparty_obj_file_with_msvc(path string, moduleflags
 		oargs << '/MDd'
 		oargs << '/D_DEBUG'
 	}
+	if env_ldflags != '' {
+		oargs << env_ldflags
+	}
 	str_oargs := oargs.join(' ')
 	cmd := '"$msvc.full_cl_exe_path" /volatile:ms $str_oargs $defines $include_string /c $cfiles /Fo"$obj_path"'
 	// NB: the quotes above ARE balanced.
@@ -429,7 +446,6 @@ fn (mut v Builder) build_thirdparty_obj_file_with_msvc(path string, moduleflags
 	if res.exit_code != 0 {
 		println('msvc: failed to build a thirdparty object; cmd: $cmd')
 		verror(res.output)
-		return
 	}
 	println(res.output)
 }

vlib/v/pref/default.v

@@ -141,8 +141,14 @@ pub fn (mut p Preferences) fill_with_defaults() {
 fn (mut p Preferences) find_cc_if_cross_compiling() {
 	if p.os == .windows {
 		$if !windows {
-			// Cross compiling to Windows
-			p.ccompiler = 'x86_64-w64-mingw32-gcc'
+			// Allow for explicit overrides like `v -showcc -cc msvc -os windows file.v`,
+			// so that the flag passing can be debugged on other OSes too, not only
+			// on windows (building will stop later, when -showcc already could display all
+			// options).
+			if p.ccompiler != 'msvc' {
+				// Cross compiling to Windows
+				p.ccompiler = 'x86_64-w64-mingw32-gcc'
+			}
 		}
 	}
 	if p.os == .linux {