CMake: respect system cflags

Adding optimization level to CMAKE_C_FLAGS is intrusive and problematic
with policies of various distribution.
However, setting "-O2" in CMAKE_CFLAGS_RELEASE is fine and only
affects release build.
CMake: fix zlib include dir

ZLIB_INCLUDE_DIR was interpreted as a relative path from the
working directory.
CMake: allow to build both shared and static at once

This allows for more fine-grained control. Possible combinations:
  * static off, shared on
  * static on, shared off
  * static on, shared on

The static library is always called "libpolarssl.a" and is only used
for linking of tests and internal programs if the shared lib is
not being built.

Default is: only build static lib.