If gpdb is configured to build with orca it requires a Greenplum patched
version of Xerces library. This commit adds an explicit check for the
patched Xerces.

This also gives us an opportunity to be more idiomatic in doing
autoconf: we are replacing the manual manipulation of LIBS in
src/backend/Makefile with `AC_CHECK_LIB` in `configure.in`.

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.

Allow additional thread flags to be added via port templates.

Change thread flag names to PTHREAD_CFLAGS and PTHREAD_LIBS to match new
configure script.

$Header: -> $PostgreSQL Changes ...

configuration.

calls with new or now-built-in versions.  Make sure that all
calls to AC_DEFINE have a third argument, for possible use of
autoheader in the future.

platforms and without relinking.

Also support VPATH builds and DESTDIR installs.  One hopes.

Ok, I've split todays commit into three, the first two already done had some
bits in JDBC & the first set of tools into contrib.

This is the third, and deals with enabling JDBC to be compiled with the main
source.

What it does is add a new option to configure: --with-java

This option tells configure to look for ant (our build tool of choice) and
if found, it then compiles both the JDBC driver and the new tools as part
of the normal make.

Also, when the postgresql install is done, all the .jar files are also
installed into the ${PGLIB}/java directory (thought best to keep then separate)

Now I had some conflicts when this applied so could someone please double check
that everything is ok?

Peter

standard installation layout it should be possible to build the HTML
and print documentation without additional manual setup.

flags for Solaris.  The test itself is straight from libtool.

add --without-tk option to disable Tk.  We don't need the AC_PATH_XTRA
test because tkConfig.sh already contains all the information about how to
compile and link with X.  Also make sure that libpq is up to date for
libpgtcl.  Remove executable bits from pgaccess.sh, but add it to pgaccess.

that's actually useful, robust, consistent.

Better plan to generate aclocal.m4 as well: use m4 include directives,
rather than cat.

incarnations (I hope). When an acceptable flex version is not found, print
instructive error messages from both configure and the makefiles, so that
users can continue building anyway.

the harm potential outweighs the possible benefits.

argument, change the order of tests for the third argument to be safe
against missing prototypes, and make it fail hard if none of the
combinations succeed.

we'll get there one day.

Use `cat' to create aclocal.m4, not `aclocal'. Some people don't
have automake installed.

Only run the autoconf rule in the top-level GNUmakefile if the
invoker specified `make configure', don't run it automatically
because of CVS timestamp skew.

Interfaced a lot of the custom tests to the config.cache, in the process
made them separate macros and grouped them out into files. Made naming
adjustments.

Removed a couple of useless/unused configure tests.

Disabled C++ by default. C++ is no more special than Perl, Python, and Tcl.
And it breaks equally often. :(

that now functions as a wrapper around the MakeMaker stuff. It might
even behave sensically when we have separate build dirs. Same for plperl,
which of course still doesn't work very well. Made sure that plperl
respects the choice of --libdir.

Added --with-python to automatically build and install the Python interface.
Works similarly to the Perl5 stuff.

Moved the burden of the distclean targets lower down into the source tree.
Eventually, each make file should have its own.

Added automatic remaking of makefiles and configure. Currently only for the
top-level because of a bug(?) in Autoconf. Use GNU `missing' to work around
missing autoconf and aclocal. Start factoring out macros into their own
config/*.m4 files to increase readability and organization.