Allow redirection of the downloads folder with an environment variable. (#4883)

* Add detection for VCPKG_DOWNLOADS environment variable in vcpkgpaths.cpp.

* Pass the downloads directory from VcpkgPaths to cmake.

* Also fixup bootstrap on *nix.

* Make error message a little prettier.

* Make that bash script actually work :)

* [vcpkg] Alter Optional<> usage style

* [vcpkg-docs] Add section on Environment Variables to the docs

2 files changed, 24 insertions, 21 deletions

diff --git a/docs/index.md b/docs/index.md
index 69dbdcd1e..cde241e92 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -2,8 +2,6 @@
 
 Vcpkg helps you manage C and C++ libraries on Windows, Linux and MacOS. This tool and ecosystem are constantly evolving; your involvement are vital to its success!
 
-- [Installing and Using Packages Example: sqlite](examples/installing-and-using-packages.md)
-
 ### Examples
 
 - [Installing and Using Packages Example: sqlite](examples/installing-and-using-packages.md)
@@ -15,6 +13,7 @@ Vcpkg helps you manage C and C++ libraries on Windows, Linux and MacOS. This too
 
 - [Integration with build systems](users/integration.md)
 - [Triplet files](users/triplets.md)
+- [Configuration and Environment](users/config-environment.md)
 
 ### Maintainer help
 
@@ -24,6 +23,7 @@ Vcpkg helps you manage C and C++ libraries on Windows, Linux and MacOS. This too
 ### Specifications
 
 - [Export](specifications/export-command.md)
+- [Feature Packages](specifications/feature-packages.md)
 
 ### Blog posts
 - [Announcing a single C++ library manager for Linux, macOS and Windows: Vcpkg](https://blogs.msdn.microsoft.com/vcblog/2018/04/24/announcing-a-single-c-library-manager-for-linux-macos-and-windows-vcpkg/)
diff --git a/docs/users/triplets.md b/docs/users/triplets.md
index 9ff372aed..4f23c8e27 100644
--- a/docs/users/triplets.md
+++ b/docs/users/triplets.md
@@ -33,24 +33,6 @@ Valid options include any CMake system name, such as:
 - `Darwin` (Mac OSX)
 - `Linux` (Linux)
 
-### VCPKG_PLATFORM_TOOLSET
-Specifies the VS-based C/C++ compiler toolchain to use.
-
-This can be set to `v141`, `v140`, or left blank. If left blank, we select the latest compiler toolset available on your machine.
-
-Visual Studio 2015 platform toolset is `v140`  
-Visual Studio 2017 platform toolset is `v141`
-
-### VCPKG_VISUAL_STUDIO_PATH
-Specifies the Visual Studio installation to use.
-
-When unspecified, a Visual Studio instance is selected automatically, preferring Stable 2017, then Preview 2017, then 2015.
-
-The path should be absolute, formatted with backslashes, and have no trailing slash:
-```cmake
-set(VCPKG_VISUAL_STUDIO_PATH "C:\\Program Files (x86)\\Microsoft Visual Studio\\Preview\\Community")
-```
-
 ### VCPKG_CHAINLOAD_TOOLCHAIN_FILE
 Specifies an alternate CMake Toolchain file to use.
 
@@ -69,7 +51,28 @@ This option also has forms for configuration-specific and C flags:
 - `VCPKG_C_FLAGS_DEBUG`
 - `VCPKG_C_FLAGS_RELEASE`
 
-## macOS Variables
+## Windows Variables
+
+### VCPKG_PLATFORM_TOOLSET
+Specifies the VS-based C/C++ compiler toolchain to use.
+
+This can be set to `v141`, `v140`, or left blank. If left blank, we select the latest compiler toolset available on your machine.
+
+Visual Studio 2015 platform toolset is `v140`
+Visual Studio 2017 platform toolset is `v141`
+
+<a name="VCPKG_VISUAL_STUDIO_PATH"></a>
+### VCPKG_VISUAL_STUDIO_PATH
+Specifies the Visual Studio installation to use.
+
+When unspecified, a Visual Studio instance is selected automatically, preferring Stable 2017, then Preview 2017, then 2015.
+
+The path should be absolute, formatted with backslashes, and have no trailing slash:
+```cmake
+set(VCPKG_VISUAL_STUDIO_PATH "C:\\Program Files (x86)\\Microsoft Visual Studio\\Preview\\Community")
+```
+
+## MacOS Variables
 
 ### VCPKG_INSTALL_NAME_DIR
 Sets the install name used when building macOS dynamic libraries. Default value is `@rpath`. See the CMake documentation for [CMAKE_INSTALL_NAME_DIR](https://cmake.org/cmake/help/latest/variable/CMAKE_INSTALL_NAME_DIR.html) for more information.