cgen: prevent `undefined symbol: tcc_backtrace` errors with tcc and `-shared -d no_backtrace`

vlib/builtin/builtin_nix.c.v

@@ -83,13 +83,13 @@ fn print_backtrace_skipping_top_frames_linux(skipframes int) bool {
 		eprintln('Some libc implementations like musl simply do not provide it.')
 		return false
 	}
+	$if no_backtrace ? {
+		return false
+	} $else {
 		$if tinyc {
 			C.tcc_backtrace("Backtrace")
 			return false
 		}
-	$if no_backtrace ? {
-		return false
-	} $else {
 		buffer := [100]byteptr{}
 		nr_ptrs := C.backtrace(voidptr(buffer), 100)
 		if nr_ptrs < 2 {

vlib/v/builder/cc.v

@@ -497,7 +497,7 @@ fn (mut v Builder) cc() {
 			}
 		}
 	}
-	if is_cc_tcc {
+	if is_cc_tcc && 'no_backtrace' !in v.pref.compile_defines {
 		args << '-bt25'
 	}
 	// Without these libs compilation will fail on Linux

vlib/v/gen/cheaders.v

@@ -99,6 +99,24 @@ typedef int (*qsort_callback_func)(const void*, const void*);
 #include <stdio.h>  // TODO remove all these includes, define all function signatures and types manually
 #include <stdlib.h>
 
+#if defined(_WIN32) || defined(__CYGWIN__)
+	#define VV_EXPORTED_SYMBOL extern __declspec(dllexport)
+	#define VV_LOCAL_SYMBOL static
+#else
+	// 4 < gcc < 5 is used by some older Ubuntu LTS and Centos versions,
+	// and does not support __has_attribute(visibility) ...
+	#ifndef __has_attribute
+		#define __has_attribute(x) 0  // Compatibility with non-clang compilers.
+	#endif
+	#if (defined(__GNUC__) && (__GNUC__ >= 4)) || (defined(__clang__) && __has_attribute(visibility))
+		#define VV_EXPORTED_SYMBOL extern __attribute__ ((visibility ("default")))
+		#define VV_LOCAL_SYMBOL  __attribute__ ((visibility ("hidden")))
+	#else
+		#define VV_EXPORTED_SYMBOL extern
+		#define VV_LOCAL_SYMBOL static
+	#endif
+#endif
+
 #ifdef __cplusplus
 	#include <utility>
 	#define _MOV std::move

vlib/v/gen/fn.v

@@ -100,8 +100,8 @@ fn (mut g Gen) gen_fn_decl(it ast.FnDecl, skip bool) {
 				// if !(g.pref.build_mode == .build_module && g.is_builtin_mod) {
 				// If we are building vlib/builtin, we need all private functions like array_get
 				// to be public, so that all V programs can access them.
-				g.write('static ')
-				g.definitions.write('static ')
+				g.write('VV_LOCAL_SYMBOL ')
+				g.definitions.write('VV_LOCAL_SYMBOL ')
 			}
 		}
 		fn_header := if msvc_attrs.len > 0 { '$type_name $msvc_attrs ${name}(' } else { '$type_name ${name}(' }
@@ -170,7 +170,7 @@ fn (mut g Gen) gen_fn_decl(it ast.FnDecl, skip bool) {
 		if attr.name == 'export' {
 			g.writeln('// export alias: $attr.arg -> $name')
 			export_alias := '$type_name ${attr.arg}($arg_str)'
-			g.definitions.writeln('$export_alias;')
+			g.definitions.writeln('VV_EXPORTED_SYMBOL $export_alias; // exported fn $it.name')
 			g.writeln('$export_alias {')
 			g.write('\treturn ${name}(')
 			g.write(fargs.join(', '))