Set PRE_TARGETDEPS in add_dependencies.pri.

PRE_TARGETDEPS is needed so that a change to a dependency static library
causes the dependents to relink.
Distinguish between MinGW and MSVC in FileIo64BitSupport.cc.

 * Only the latter has 64-bit fseek and ftell functions.

 * I'm only considering the plain MinGW project.  I'm slightly curious as
   to whether MinGW-w64 or Clang are able to use 64-bit fseek/ftell
   functions.
MinGW: The Clang/MinGW libraries need pthread, add it to the link line.
Define SOURCEWEB_UNIX to 1 instead of nothing.

 * This makes the macro consistent with a hypothetical -DSOURCEWEB_UNIX
   command-line argument.
Implement a Win32-specific version of indexdb::MappedReader.
Ensure that NOMINMAX is always 1 before including windows.h.

 * This macro needs to be defined so that windows.h does not define min
   and max macros, which conflict with std::min and std::max.

 * This applies only to code in SourceWeb proper.
   third_party/libre2/util/mutex.h currently includes windows.h without
   defining NOMINMAX.  It does define WIN32_MEAN_AND_LEAN, so maybe that's
   good enough.

 * To improve readability, take advantage of the mutual exclusivity of
   SOURCEWEB_UNIX and _WIN32 and replace if/elif/endif with two if/endif
   blocks in a few places.  Add a INDEXER_MUTEX_USE_CXX11 macro which is
   mutually exclusive with the other INDEXER_MUTEX_USE_xxx macros.