This PR fixes a few issues I encountered while building the library:
Find optional libraries is turned to quiet, so if OpenCV or Iconv are not detected the respective modules are not built silently. Previously it would fail the compilation.
Build libzxing as a shared library, makes it easier to link against shared library (no need to build in PIC mode).
Symbols are exported correctly when compiling on Windows MSVC2017.
This PR fixes a few issues I encountered while building the library:
libzxingas a shared library, makes it easier to link against shared library (no need to build in PIC mode).