Adding support for sketch-specific build properties

[stevemarple]

Tests for existence of a build_props.txt file in the sketch
directory. If present reads properties similar to those found in the
hardware and board files, allowing the user to easily customise the
build for a specific sketch. Most useful when containing lines similar
to

compiler.c.extra_flags=-D NDEBUG
compiler.cpp.extra_flags=-D NDEBUG -D MYLIBRARY_BUFSIZE=100

which enables the user to have macros defined for all compilation
units, including libraries.

Signed-off-by: Steve Marple stevemarple@googlemail.com

[stevemarple]

One point of consideration: adding sketch-specific build properties allows the user to override build.path (which is removed when all compilation units must be rebuilt), commands such as compiler.cmd and paths such as compiler.path. Sketches and their build_props.txt file taken from untrusted sources could therefore be considered harmful. This could be mitigated by requiring a user-preference to be set before sketch-specific build properties are enabled. Alternatively the properties could be limited to a subset (eg by excluding *.path, *.cmd, and recipe.*).

[stevemarple]

Addresses #15

[ffissore]

Assigning to @cmaglie: he will explain why such external defines are considered harmful

[ffissore]

btw, nice code, thank you very much

[FrankBoesing]

ping..
nothing happened.. are you going to merge this, or not ?

[luelista]

Hey, @arduino, @ffissore, @cmaglie
... I think this pull request implements a very useful and missing feature - what do you think about merging it?

[ffissore]

@max-weller sorry you missed the news. I no longer work for arduino https://groups.google.com/a/arduino.cc/forum/#!msg/developers/YJLX6AZHem4/kpsLef7XBAAJ

[ricardojlrufino]

+1

[CLAassistant]

All committers have signed the CLA.