slapi plugins: fix CFLAGS
    
    Add explicit NSPR_CFLAGS and NSS_CFLAGS where NSPR_LIBS and NSS_LIBS is
    used.
    
    Use DIRSRV_CFLAGS rather than hardcode -I/usr/include/dirsrv.
    
    Append NSPR_CFLAGS to DIRSRV_CFLAGS in ./configure as slapi-plugin.h
    includes nspr.h.
    
    Reviewed-By: Pavel Vomacka <pvomacka@redhat.com>