+ add <cmath> header to keep cl.exe happy
Use shared pointer reference count to track pools

By using the shared pointer reference count to track the connections
in the connection pool we no longer have to explicitly return them
to the pool.

This resolves an issue where cursor feature sets were returning their
connections before they were done with them, causing a potential race
with another connection user.
Merge pull request #1823 from thjc/master

Fix for #1588 Postgis: Concurrency problem with CursorResultSet. Thanks @thjc
rename variable for clarity
mapnik::value

to_expression_string - add missing specialization
for mapnik::value_integer and remove usage of std::stringstream
remove uneeded python imports
Add test for image.fromstring - refs #1805
commment failing tiff test - refs #1831
Add tests for frombuffer as well - refs #1805
reduce libary size by passing -fvisibility-inlines-hidden - closes #1832 - refs #1826
no need to use MAPNIK_DECL with typedefs
make public the singleton template to ensure it can be shared across shared library boundaries if the library is built with -fvisibility=hidden - refs #1826
make mapped_memory_cache a class not struct for consistency with other singletons
fix build on windows by ensuring singleton base is exported (avoids inconsistent dll linkage error)
support visibility attribute with gcc/clang - refs #1826
-fvisibility=hidden works with plugins with no code changes - refs #1826
remove kismet plugin - closes #1833
upgrade clipper to 5.1.5 - refs #1749
no need to cache config error specifically
support -fvisibility=hidden in core library - refs #1826