Apply BuildDependentLibraries to src_filter dirs

[davidgraeff]

At the moment BuildDependentLibraries (scanning source files for lib
includes) is only executed for the src directory, even if src_filter
brings in other directories. This few lines of code changes apply the
scanning to all additional dirs mentioned in src_filter with a "+".

Use cases

Have targets in your platformio.ini which references additional source directories via the src_filter.

1. In my case i want to target an arduino and an esp8266 board. The core logic
   of my library is platform independant but some few files are board specific, so I need
   to pull them in via src_filter (which works great). This pull request allows mentioned
   libraries in those additional files to be handled like if the source files reside in "src/".

2. If you provide some examples with a library, at the moment it is difficult to provide the user
   a simple way to build an example. With this pull request the user just builds one of the
   targets in the platformio.ini which lists the example source files via src_filter. E.g.:

`[platformio]
src_dir = src/

[env:nodemcuv2lib]
platform = espressif
framework = arduino
board = nodemcuv2

[env:nodemcuv2Example1]
platform = espressif
framework = arduino
board = nodemcuv2
src_filter = +<> +<../examples/example1/src/>`

[ivankravets]

Thank you for this pull request.

1. You can write multiplatform code using different macros. PlatformIO passes these defines by default. For example, -DESP8266, -DARDUINO_ARCH_AVR and then #ifdef... #endif.
2. Take a look at platformio ci command. You have the ability to run library example and pass the source code of library to build process.
3. We have a separate issue with extra options for the Library Builder. See Handle specific extra build flags from library.json #289

[davidgraeff]

@ivankravets Thanks for the fast response.

1. Handle specific extra build flags from library.json #289 is good thing indeed, will it be pio2 or be part of pio3?

2. platformio ci with the "-l" options is what I was looking for. On linux I made a symlink, that wasn't possible on windows machines. Is there already support in the atom based IDE for this workflow?

3. I still thing that "src" may either be an array with directories or src_filter added directories are handled
   in a similar fashion to the "src" dir.
   Sometimes the directory structure cannot be that simple. In my case one "platform" directory is a git submodule and I cannot easily add #ifdefs to the files, but the src_filter option would allow me to use this workflow.

[ivankravets]

1. Handle specific extra build flags from library.json #289 is a part of PlatformIO 3.0.
2. I don't recommend to mix "program source code" with "library source code". The aim of PlatformIO 3.0 is to make these components maximum isolated. As a result, we will introduce additional leverages to control build process for the libraries.

I propose to wait for the first PlatformIO 3.0beta and then we will discuss it.

In any case, thanks a lot for the PR and I'll not close it till final PlatformIO 3.0.

[davidgraeff]

@ivankravets ok then. i can live with my patched version of pio2 until pio3 is out