ecpg_config.h symbol missing with meson

I checked the generated ecpg_config.h with make and meson, and the meson 
one is missing

#define HAVE_LONG_LONG_INT 1

This is obviously quite uninteresting, since that is required by C99. 
But it would be more satisfactory if we didn't have discrepancies like 
that.  Note that we also kept ENABLE_THREAD_SAFETY in ecpg_config.h for 
compatibility.

Fixing this on the meson side would be like

diff --git a/src/interfaces/ecpg/include/meson.build 
b/src/interfaces/ecpg/include/meson.build
index 31610fef589..b85486acbea 100644
--- a/src/interfaces/ecpg/include/meson.build
+++ b/src/interfaces/ecpg/include/meson.build
@@ -12,6 +12,7 @@ ecpg_conf_keys = [
  ecpg_conf_data = configuration_data()

  ecpg_conf_data.set('ENABLE_THREAD_SAFETY', 1)
+ecpg_conf_data.set('HAVE_LONG_LONG_INT', 1)

  foreach key : ecpg_conf_keys
    if cdata.has(key)

Alternatively, we could remove the symbol from the make side.