Each recipe consists of 3 main parts: defining identifiers, setting build
variables, and defining build commands.

The package "mylib" will be used here as an example

General tips:
- mylib_foo is written as $(package)_foo in order to make recipes more similar.

## Identifiers
Each package is required to define at least these variables:

```
$(package)_version:
Version of the upstream library or program. If there is no version, a
placeholder such as 1.0 can be used.

$(package)_download_path:
Location of the upstream source, without the file-name. Usually http or
ftp.

$(package)_file_name:
The upstream source filename available at the download path.

$(package)_sha256_hash:
The sha256 hash of the upstream file
```

These variables are optional:

```
$(package)_build_subdir:
cd to this dir before running configure/build/stage commands.

$(package)_download_file:
The file-name of the upstream source if it differs from how it should be
stored locally. This can be used to avoid storing file-names with strange
characters.

$(package)_dependencies:
Names of any other packages that this one depends on.

$(package)_patches:
Filenames of any patches needed to build the package

$(package)_extra_sources:
Any extra files that will be fetched via $(package)_fetch_cmds. These are
specified so that they can be fetched and verified via 'make download'.
```


## Build Variables:
After defining the main identifiers, build variables may be added or customized
before running the build commands. They should be added to a function called
$(package)_set_vars. For example:

```
define $(package)_set_vars
...
endef
```

Most variables can be prefixed with the host, architecture, or both, to make
the modifications specific to that case. For example:

```
Universal:     $(package)_cc=gcc
Linux only:    $(package)_linux_cc=gcc
x86_64 only:       $(package)_x86_64_cc = gcc
x86_64 linux only: $(package)_x86_64_linux_cc = gcc
```

These variables may be set to override or append their default values.

```
$(package)_cc
$(package)_cxx
$(package)_objc
$(package)_objcxx
$(package)_ar
$(package)_ranlib
$(package)_libtool
$(package)_nm
$(package)_cflags
$(package)_cxxflags
$(package)_ldflags
$(package)_cppflags
$(package)_config_env
$(package)_build_env
$(package)_stage_env
$(package)_build_opts
$(package)_config_opts
```

The `*_env` variables are used to add environment variables to the respective
commands.

Many variables respect a debug/release suffix as well, in order to use them for
only the appropriate build config. For example:

```
$(package)_cflags_release = -O3
$(package)_cflags_i686_debug = -g
$(package)_config_opts_release = --disable-debug
```

These will be used in addition to the options that do not specify
debug/release. All builds are considered to be release unless DEBUG=1 is set by
the user. Other variables may be defined as needed.

## Build commands:

For each build, a unique build dir and staging dir are created. For example,
`work/build/mylib/1.0-1adac830f6e` and `work/staging/mylib/1.0-1adac830f6e`.

The following build commands are available for each recipe:

```
$(package)_fetch_cmds:
Runs from: build dir
Fetch the source file. If undefined, it will be fetched and verified
against its hash.

$(package)_extract_cmds:
Runs from: build dir
Verify the source file against its hash and extract it. If undefined, the
source is assumed to be a tarball.

$(package)_preprocess_cmds:
Runs from: build dir/$(package)_build_subdir
Preprocess the source as necessary. If undefined, does nothing.

$(package)_config_cmds:
Runs from: build dir/$(package)_build_subdir
Configure the source. If undefined, does nothing.

$(package)_build_cmds:
Runs from: build dir/$(package)_build_subdir
Build the source. If undefined, does nothing.

$(package)_stage_cmds:
Runs from: build dir/$(package)_build_subdir
Stage the build results. If undefined, does nothing.
```

The following variables are available for each recipe:

```    
$(1)_staging_dir: package's destination sysroot path
$(1)_staging_prefix_dir: prefix path inside of the package's staging dir
$(1)_extract_dir: path to the package's extracted sources
$(1)_build_dir: path where configure/build/stage commands will be run
$(1)_patch_dir: path where the package's patches (if any) are found
```

Notes on build commands:

For packages built with autotools, `$($(package)_autoconf)` can be used in the
configure step to (usually) correctly configure automatically. Any
`$($(package)_config_opts`) will be appended.

Most autotools projects can be properly staged using:

```bash
$(MAKE) DESTDIR=$($(package)_staging_dir) install
```
