Fix build on ancient versions of gcc

Default on gcc 4.6 was 'gnu90', which allows some c99 features
but not 'for (int n = 0, ...)'

Adds a -std=gnu90 build to the tests so we don't get caught by
this again.

.cirrus.yml

@@ -137,6 +137,7 @@ rocky_task:
     LC_ALL: C
     CIRRUS_CLONE_DEPTH: 1
     USE_CONFIG: yes
+    CFLAGS: -std=gnu90
 
   # NB: we could consider building a docker image with these
   # preinstalled and specifying that instead, to speed up testing.

vcf.c

@@ -5097,6 +5097,7 @@ static int add_desc_to_buffer(char *buffer, size_t *offset, size_t maxbuffer, co
 const char *bcf_strerror(int errorcode, char *buffer, size_t maxbuffer) {
     size_t usedup = 0;
     int ret = 0;
+    int idx;
 
     if (!buffer || maxbuffer < 4)
         return NULL;           //invalid / insufficient buffer
@@ -5106,7 +5107,7 @@ const char *bcf_strerror(int errorcode, char *buffer, size_t maxbuffer) {
         return buffer;
     }
 
-    for (int idx = 0; idx < sizeof(errdesc_bcf) / sizeof(err_desc); ++idx) {
+    for (idx = 0; idx < sizeof(errdesc_bcf) / sizeof(err_desc); ++idx) {
         if (errorcode & errdesc_bcf[idx].errorcode) {    //error is set, add description
             ret = add_desc_to_buffer(buffer, &usedup, maxbuffer, errdesc_bcf[idx].description);
             if (ret < 0)