Add wget as a dependency

Summary:
Closes https://github.com/facebook/proxygen/pull/5
Github Author: Gabriele Carrettoni <gabriele.carrettoni@gmail.com>

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Reviewers: dcsommer

Reviewed By: dcsommer
Guard against older versions of OpenSSL

Summary:
ServerNameCallbackResult isn't always defined by thrift
if the OpenSSL version is too old.

Test Plan: HHVM OSS build

Reviewers: dcsommer, afrind

Reviewed By: afrind
Allow building without gflags

Summary:
NO_LIB_GFLAGS is a define we've used elsewhere
to deal with not having gflags on the local system

Test Plan: HHVM OSS build

Reviewers: dcsommer, afrind

Reviewed By: afrind
revert the window update diff

Reviewed By: dcsommer
Add cancel to error enum

Reviewers: seanc, ranjeeth, dcsommer, subodh

Reviewed By: subodh
Use static buffer for mobile builds
Build system improvements.

Summary:
Separate dependency fetching + building from installing.
Closes https://github.com/facebook/proxygen/pull/8
Github Author: Daniel Sommermann <dcsommer@gmail.com>

{sync, type="child", parent="github", parentrepo="facebook/proxygen", parentprnum="8", parentprfbid="843969238967487"}

Test Plan: unit tests. tested on my laptop.

Reviewers: davejwatson, viswanath, praveenr

Reviewed By: praveenr
Update README.md with blog post link

Reviewed By: cgheorghe
Build system improvements.

Summary:
Separate dependency fetching + building from installing.
Closes https://github.com/facebook/proxygen/pull/8
Github Author: Daniel Sommermann <dcsommer@gmail.com>

{sync, type="child", parent="github", parentrepo="facebook/proxygen", parentprnum="8", parentprfbid="843969238967487"}

Test Plan: unit tests

Reviewers: davejwatson, viswanath, praveenr

Reviewed By: praveenr
Fixed minor typo

Summary:
Closes https://github.com/facebook/proxygen/pull/6
Github Author: Pascal Borreli <pascal@borreli.com>

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Test Plan: Imported from GitHub, without a `Test Plan:` line.

Reviewers: viswanath, afrind, cgheorghe, praveenr

Reviewed By: praveenr