BUILD: fix a warning emitted by isblank() on non-c99 compilers

BUILD: fix a warning emitted by isblank() on non-c99 compilers

Commit a2b9dad introduced use of isblank() which is not present everywhere
and requires -std=c99 or various other defines. Here on gcc-3.4 with glibc
2.3 it emits a warning though it works :

  src/checks.c: In function 'event_srv_chk_r':
  src/checks.c:1007: warning: implicit declaration of function 'isblank'

This macro matches only 2 values, better replace it with the explicit match.