Fix pkgconfig files on windows

[nirbheek]

build: Fix libdir in pkgconfig files for Windows

Windows import libraries are installed inside ARCHIVE_DIRECTORY, not
LIBRARY_DIRECTORY. So we need to use that for libdir on Windows.

build: Set -DUSE_IMPORT_EXPORT in pkgconfig files on Windows

MSVC consumers of the shared library need to define this macro when
linking to aws-cpp-sdk-* libs, or they will get undefined reference
errors. Also doesn't hurt to define this with MinGW, so add it
unconditionally on Windows.

This PR plus https://github.com/amzn/amazon-s3-gst-plugin/pull/23 fixes https://github.com/amzn/amazon-s3-gst-plugin/issues/13

Check all that applies:

• [x] Did a review by yourself.
• [ ] Added proper tests to cover this PR. (If tests are not applicable, explain.)
  • Build system fix, doesn't require a test
• [x] Checked if this PR is a breaking (APIs have been changed) change.
• [x] Checked if this PR will not introduce cross-platform inconsistent behavior.
• [x] Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• [x] Linux
• [x] Windows
• [ ] Android
• [ ] MacOS
• [ ] IOS
• [ ] Other Platforms

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[nacho]

LGTM

[nirbheek]

Could someone please review this? It is necessary for an Amazon product to work correctly :)

[nirbheek]

Poke?

[jmklix]

#1888

[jawilson]

Poke?