Emit a warning at compile time about C++ API being unstable

Aware users can remove the warning by defining macro
USE_UNSTABLE_GEOS_CPP_API

See discussion
https://lists.osgeo.org/pipermail/geos-devel/2017-October/008054.html

configure.ac

@@ -39,6 +39,10 @@ AM_MAINTAINER_MODE
 AC_CONFIG_HEADERS([include/config.h] [include/geos/platform.h])
 AC_PROG_CC
 
+
+dnl Hush warnings
+AC_DEFINE(USE_UNSTABLE_GEOS_CPP_API, [1], [We know])
+
 dnl use libtool ----------------------------------------------------------
 AC_LIBTOOL_DLOPEN
 AC_LIBTOOL_WIN32_DLL
@@ -162,7 +166,8 @@ AC_LIBTOOL_COMPILER_OPTION([if $compiler supports -Wno-long-long], [dummy_cv_wno
 NUMERICFLAGS=""
 AC_LIBTOOL_COMPILER_OPTION([if $compiler supports -ffloat-store], [dummy_cv_ffloat_store], [-ffloat-store], [], [NUMERICFLAGS="$NUMERICFLAGS -ffloat-store"], [])
 
-DEFAULTFLAGS="${WARNFLAGS} ${NUMERICFLAGS}"
+HUSHWARNING="-DUSE_UNSTABLE_GEOS_CPP_API"
+DEFAULTFLAGS="${WARNFLAGS} ${NUMERICFLAGS} ${HUSHWARNING}"
 
 AM_CXXFLAGS="${AM_CXXFLAGS} ${DEFAULTFLAGS}"
 AM_CFLAGS="${AM_CFLAGS} ${DEFAULTFLAGS}"

include/geos/geom/Geometry.h

@@ -21,6 +21,11 @@
 #ifndef GEOS_GEOM_GEOMETRY_H
 #define GEOS_GEOM_GEOMETRY_H
 
+#ifndef USE_UNSTABLE_GEOS_CPP_API
+# warning "The GEOS C++ API is unstable, please use the C API instead"
+# warning "HINT: #include geos_c.h"
+#endif
+
 #include <geos/export.h>
 #include <geos/platform.h>
 #include <geos/inline.h>

tests/xmltester/Makefile.am

@@ -136,10 +136,6 @@ XMLTester_SOURCES = \
 
 XMLTester_LDADD = $(LIBS)
 
-# Intentionally drop -ansi -pedantic
-# See http://trac.osgeo.org/geos/ticket/319
-XMLTester_CXXFLAGS = $(INLINE_FLAGS)
-
 AM_CPPFLAGS = -I$(top_srcdir)/include
 AM_CPPFLAGS += -I$(top_srcdir)/src/io/tinyxml -DTIXML_USE_STL