Clarify problems with linking against C++ api (libgeos#553)

Path by Greg Troxel.

git-svn-id: http://svn.osgeo.org/geos/trunk@3663 5242fede-7e19-0410-aef8-94bd7d2200fb

README

@@ -57,6 +57,15 @@ Example usage:
 Using the C++ interface (discouraged)
 -------------------------------------
 
+NB: The C++ interface should not be used directly; the geos project
+views it as a bug for another program to use the C++ interface or even
+to directly link against the C++ library.  The C++ library name will
+change on every minor release because it is too hard to know if there
+have been ABI changes.  (The C libary uses the C++ interface, but the
+C library follows normal ABI-change-sensitive versioning, so programs
+that link only against the C library should work without relinking
+when geos is upgraded.)
+
 To compile programs against the C++ lib:
 
   CFLAGS += `geos-config --cflags`

src/Makefile.am

@@ -21,6 +21,10 @@ INCLUDES = -I$(top_srcdir)/include
 
 lib_LTLIBRARIES = libgeos.la
 
+# libgeos uses -release because it is not feasible to know if the ABI
+# has changed between releases; the project chooses not to expend the
+# effort to determine this because depending programs should not be
+# using the C++ library.
 libgeos_la_LDFLAGS = \
     -release @VERSION_MAJOR@.@VERSION_MINOR@.@VERSION_PATCH@ \
     -no-undefined