Silence compiler warnings about array subscripts of type 'char'

Cygwin GCC complains about code like isprint(c), where c is of type char.
The isX() and toX() functions/macros (ISO C allows either) all accept an
int, whose value should be either that of an unsigned char, or the
special value EOF (== -1).
So cast to unsigned char each argument to isprint, tolower, etc.

Silences several warnings of the form:

gpsutils.c: In function 'safe_atof':
gpsutils.c:90:5: warning: array subscript has type 'char'
[-Wchar-subscripts]
      while (isspace(*p)) {
      ^
gpsutils.c:188:2: warning: array subscript has type 'char'
[-Wchar-subscripts]
   while (isdigit(*p)) {
   ^

1 files changed, 2 insertions, 2 deletions

diff --git a/gpsutils.c b/gpsutils.c
index 9b0761b2..dfcaa3f9 100644
--- a/gpsutils.c
+++ b/gpsutils.c
@@ -87,7 +87,7 @@ double safe_atof(const char *string)
      */
 
     p = string;
-    while (isspace(*p)) {
+    while (isspace((unsigned char) *p)) {
 	p += 1;
     }
     if (*p == '-') {
@@ -185,7 +185,7 @@ double safe_atof(const char *string)
 	    }
 	    expSign = false;
 	}
-	while (isdigit(*p)) {
+	while (isdigit((unsigned char) *p)) {
 	    exp = exp * 10 + (*p - '0');
 	    p += 1;
 	}