doc: Documentation in Markdown for Depends Dir

Documentation more readable when viewed on Github.

Some extra changes by @laanwj:

- Make README.usage the default README. This is more convenient from a
  user perspective. Link to other documentation in this default README

- Add list of popular targets for cross compilation, change default to
  Win64 instead of Win32

1 files changed, 147 insertions, 0 deletions

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+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:
+
+    $(MAKE) DESTDIR=$($(package)_staging_dir) install