Fix windows library install directory

[RReichert]

Issue

see https://snav.slack.com/archives/CG9PUMGUX/p1627996673000900

Testing

Ran the installation on my Linux machine with MingGW cross compilation and got the following results (need to specify the -D BUILD_SHARED_LIBS=true options for cmake):

====================[ Install | release-windows-x86_64 ]========================
/opt/jetbrains/clion/bin/cmake/linux/bin/cmake --build /home/rodrigor/Workspace/swift-nav/libsbp/c/build/release/windows-x86_64 --target install
[0/1] Install the project...
-- Install configuration: "Release"
-- Installing: /home/rodrigor/Workspace/swift-nav/libsbp/install/release/windows-x86_64/usr/x86_64-w64-mingw32/usr/lib/libsbp.dll.a
-- Installing: /home/rodrigor/Workspace/swift-nav/libsbp/install/release/windows-x86_64/usr/x86_64-w64-mingw32/usr/bin/libsbp.dll
-- Installing: /home/rodrigor/Workspace/swift-nav/libsbp/install/release/windows-x86_64/usr/x86_64-w64-mingw32/usr/include/libsbp
...

Cherry picked the changes and ran on the ESD-2043-portability-project branch with VS2019 and got the following (firstly the master branch currently doesn't compiler with VS2019, also I needed to disable the unit tests as the post build tests were failing to link to gtest.dll):

====================[ Install | Debug ]=========================================
"C:\Program Files\JetBrains\CLion 2021.1.2\bin\cmake\win\bin\cmake.exe" --build C:\Repo\libsbp\c\build\debug\vs2019 --target install
[100%] Built target sbp
Install the project...
-- Install configuration: "Debug"
-- Up-to-date: C:/Program Files (x86)/libsbp/lib/sbp.lib
-- Installing: C:/Program Files (x86)/libsbp/bin/sbp.dll
-- Up-to-date: C:/Program Files (x86)/libsbp/include/libsbp
...