Add scripts to generate Debian packages of Fluoride dependencies

via a package manager. You may have to build these from source and install them

to your local environment.

TODO(abhishekpandit) - Provide a pre-packaged option for these or proper build

instructions from source.

* libchrome

* modp_b64

* tinyxml2

We provide a script to produce debian packages for those components, please

follow the instructions in build/dpkg/README.txt.

The googletest packages provided by Debian/Ubuntu (libgmock-dev and

libgtest-dev) do not provide pkg-config files, so you can build your own

googletest using the steps below:

```

$ git clone https://github.com/google/googletest.git -b release-1.10.0

$ cd googletest        # Main directory of the cloned repository.

$ mkdir build          # Create a directory to hold the build output.

$ cd build

$ cmake ..             # Generate native build scripts for GoogleTest.

$ sudo make install -DCMAKE_INSTALL_PREFIX=/usr

```

### Stage your build environment

This directory contains scripts to build Debian packages as dependencies for

building Fluoride on Linux.

To generate the Debian packages, you need:

* Debian 10 (Buster)

* gn, get binary from https://chrome-infra-packages.appspot.com/dl/gn/gn/linux-amd64/+/latest

* apt-get install git debmake

* Other build dependencies are package dependent, you will get error message

  mentioning the list of missing packages

Steps to build debian packages (modp_b64 first):

```

$ cd build/dpkg/modp_b64

$ ./gen-src-pkg /tmp/modp_b64

```

If the above succeeded, there will be a .deb file in /tmp/modp_b64. You can

install this binary package locally like:

```

$ sudo dpkg -i /tmp/modp_b64/modp-b64_0.0.1-1_amd64.deb

```

After installing modp_b64, you can do the same steps with libchrome in

build/dpkg/libchrome.

libchrome for Debian

libchrome (780652-1) buster; urgency=low

  * Initial release.

 -- Sonny Sasaka <sonnysasaka@chromium.org>  Fri, 19 Mar 2021 19:41:40 +0000

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