Summary:
* Properly propagate the C++ compiler from autoconf
* Disable aggressive loop optimizations in GCC 4.8+

This commit is in the spirit of postgres/postgres@105f3ef4 , but done
for C++ code so that the GPORCA translator can properly build when the
compiler is GCC 4.8+

Fixes greenplum-db/gpdb#1186 .

TL;DR, here is the additional context on what specific to GPORCA translator:

GCC 6.2.0 compiler (which is required for codegen) got aggressive loop optimization to remove loop when enumerating variable length array.

In code `CTranslatorRelcacheToDXL.cpp`, we have a loop to enumerate all
the attributes of an index:

```
INT iAttno = pgIndex->indkey.values[ul];
```

Where `values[ul]` is of following data structure:

```
typedef struct
{
...
	int2		values[1];		/* VARIABLE LENGTH ARRAY */
} int2vector;					/* VARIABLE LENGTH STRUCT */
```

This is standard way of dealing with variable length array in Postgres. However, optimizer
thinking there is only 1 element, hence removed the loop.

This caused wrong result when using GPDB with GPORCA. The repro is
```
-- multi-column index
create table index_test (a int, b int, c int, d int, e int,
  constraint index_test_pkey PRIMARY KEY (a, b, c, d));

insert into index_test select i,i%2,i%3,i%4,i%5 from generate_series(1,100) i;

-- expecting using index scan with index cond: c=5
explain select * from index_test where c = 5;
```

When compile with GCC 6.2.0, the `index scan` is not selected, but a `table scan` is chosen
instead. The root cause is the compiler removed the loop, and only keep the first column.

The fix is to add $CXXFLAG auto detection to add the
`-fno-aggressive-loop-optimizations` for $CXX compilers if that version of compiler supporting
such optimization. Hence to avoid having the above issue for gpopt component.