Fixed build issues on window

[oontvoo]

• Added missing dlimport/export attributes in function definitions. (They are needed in both decls and defs)
• Removed dlimport/dlexprt attribute in private field. (global_context is not exported anywhere).

[oontvoo]

@dominichamon : more fixes :)

[dmah42]

":(.text+0x2e7e): undefined reference to 'benchmark::internal::global_context[abi:cxx11]'" suggests we do need something on global_context

[oontvoo]

global_context

Oh, turns out - there are a few places in the code where this variable was forward-declared then used :(
I'll move the decl to the header then - IMO, it's not very clean to use forward decl like this.

[sergiud]

In general, benchmark exports too many internal symbols already. Consider compiling the internal parts as object files that are reused for tests only. This avoids exposing internal symbols in the library which reduces its size. See, for example, how this is solved in Ceres solver.

[dmah42]

In general, benchmark exports too many internal symbols already. Consider compiling the internal parts as object files that are reused for tests only. This avoids exposing internal symbols in the library which reduces its size. See, for example, how this is solved in Ceres solver.

that's really clean.

i'm going to merge this to resolve the current issues but a longer term plan to move things to internal library sounds great.