cygwin 1.7 build fixes

It's less accepting of signed char ... insisting that e.g. tolower()
not receive one as a parameter.

It's probably good to phase out such usage, given the number of bugs
that lurk in the vicinity (assumptions that char is unsigned), so fix
these even though such usage is actually legal.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

src/svf/svf.c

@@ -470,7 +470,8 @@ free_all:
 #define SVFP_CMD_INC_CNT			1024
 static int svf_read_command_from_file(int fd)
 {
-	char ch, *tmp_buffer = NULL;
+	unsigned char ch;
+	char *tmp_buffer = NULL;
 	int cmd_pos = 0, cmd_ok = 0, slash = 0, comment = 0;
 
 	while (!cmd_ok && (read(fd, &ch, 1) > 0))