Add Package Manager C/C++ Language Standard Support proposal

Author: aciidgh

proposals/NNNN-package-manager-cpp-language-version.md

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+# Package Manager C/C++ Language Standard Support
+
+* Proposal: SE-NNNN
+* Authors: [Ankit Aggarwal](https://github.com/aciidb0mb3r)
+* Review Manager: [Daniel Dunbar](https://github.com/ddunbar)
+* Status: **Discussion**
+
+## Introduction
+
+This proposal adds support for declaring the language standard for C and C++
+targets in a SwiftPM package.
+
+## Motivation
+
+The C++ language standard is one of the most important build setting needed to
+compile C++ targets. We want to add some mechanism to declare it until we get
+the complete build settings feature, which is deferred from the Swift 4 release.
+
+## Proposed solution
+
+We will add support to the package manifest declaration to specify the C and C++
+language standards:
+
+```swift
+let package = Package(
+    name: "CHTTP",
+    ...
+    cStandard: .c89,
+    cxxStandard: .cxx11
+)
+```
+
+These settings will apply to all the C and C++ targets in the package. The
+default value of these properties will be `nil`, i.e. language standard flag
+will not be passed when invoking the C/C++ compiler.
+
+_Once we get the build settings feature, we will deprecate these properties._
+
+## Detailed design
+
+The C/C++ language standard will be defined by the enums below and
+updated as per the Clang compiler [repository](https://github.com/llvm-mirror/clang/blob/master/include/clang/Frontend/LangStandards.def).
+
+```swift
+public enum CLanguageStandard {
+    case c89
+    case c90
+    case iso9899_1990
+    case iso9899_199409
+    case gnu89
+    case gnu90
+    case c99
+    case iso9899_1999
+    case gnu99
+    case c11
+    case iso9899_2011
+    case gnu11
+}
+
+public enum CXXLanguageStandard {
+    case cxx98
+    case cxx03
+    case gnucxx98
+    case gnucxx03
+    case cxx11
+    case gnucxx11
+    case cxx14
+    case gnucxx14
+    case cxx1z
+    case gnucxx1z
+}
+```
+## Impact on existing code
+
+There will be no impact on existing packages because this is a new API and the
+default behaviour remains unchanged.
+
+## Alternatives considered
+
+We considered adding this property at target level but we think that will
+pollute the target namespace. Moreover, this is a temporary measure until we get
+the build settings feature.