qcow_utils: add pkgconfig script

Add a pkgconfig file and script to fix it up at build time.

Also fix extern "C" for the qcow_utils header.

BUG=chromium:806119
TEST=emerge-eve-kvm crosvm

Change-Id: Ib69d9e88b42d2f2c8661798c37537a4236e0506e
Reviewed-on: https://chromium-review.googlesource.com/891572
Commit-Ready: Stephen Barber <smbarber@chromium.org>
Tested-by: Stephen Barber <smbarber@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

qcow_utils/libqcow_utils.pc.in

@@ -0,0 +1,8 @@
+bslot=@BSLOT@
+include_dir=@INCLUDE_DIR@
+
+Name: libqcow_utils
+Description: QCOW2 shared library.
+Version: ${bslot}
+CFlags: -I${include_dir}
+Libs: -lqcow_utils

qcow_utils/platform2_preinstall.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+# Copyright 2018 The Chromium OS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+set -e
+
+v=$1
+include_dir=$2
+target_dir=$3
+
+sed \
+  -e "s/@BSLOT@/${v}/g" \
+  -e "s:@INCLUDE_DIR@:${include_dir}:g" \
+  "qcow_utils/libqcow_utils.pc.in" > "${target_dir}/libqcow_utils.pc"

qcow_utils/src/qcow_utils.h

@@ -6,5 +6,13 @@
 
 // Exported interface to basic qcow functionality to be used from C.
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 // Create a basic, empty qcow2 file that can grow to `virtual_size` at `path`.
 int create_qcow_with_size(const char *path, uint64_t virtual_size);
+
+#ifdef __cplusplus
+};
+#endif