make: strip leading whitespace in shell assignment

Assignment of shell output to a macro ('!=') was originally a
non-POSIX extension.  It later became a POSIX 202X feature.

However, the implementation failed to include the additional
POSIX requirement that leading whitespace is removed from the
shell output.

Neither GNU make nor bmake strip leading whitespace.  Implement
this behaviour as a non-POSIX extension.

miscutils/make.c

@@ -1636,22 +1636,39 @@ run_command(const char *cmd)
 	}
 	pclose(fd);
 
-	if (val) {
-#if ENABLE_PLATFORM_MINGW32
-		len = remove_cr(val, len + 1) - 1;
+	if (val == NULL)
+		return NULL;
+
+	// Strip leading whitespace in POSIX 202X mode
+	if (posix) {
+		s = val;
+		while (isspace(*s)) {
+			++s;
+			--len;
+		}
+
 		if (len == 0) {
 			free(val);
 			return NULL;
 		}
+		memmove(val, s, len + 1);
+	}
+
+#if ENABLE_PLATFORM_MINGW32
+	len = remove_cr(val, len + 1) - 1;
+	if (len == 0) {
+		free(val);
+		return NULL;
+	}
 #endif
-		// Remove one newline from the end (BSD compatibility)
-		if (val[len - 1] == '\n')
-			val[len - 1] = '\0';
-		// Other newlines are changed to spaces
-		for (s = val; *s; ++s) {
-			if (*s == '\n')
-				*s = ' ';
-		}
+
+	// Remove one newline from the end (BSD compatibility)
+	if (val[len - 1] == '\n')
+		val[len - 1] = '\0';
+	// Other newlines are changed to spaces
+	for (s = val; *s; ++s) {
+		if (*s == '\n')
+			*s = ' ';
 	}
 	return val;
 }