remove libsass.js upon `make clean`
fixing `make install` (install -D) on OSX

`install -D` does not work on OSX, but since it only creates the target directories, it can easily be split out into a `mkdir -p …`.

```
install -Dpm0755 libsass.a /usr/local/lib/libsass.a
install: illegal option -- D
```
include cstdlib for std::abs() for Emscripten

While gcc compiled fine, emscripten did not. The following error was thrown:

```
/usr/local/bin/emscripten/emscripten/1.8.2/em++ -Wall -O2 -fPIC -c -o utf8_string.o utf8_string.cpp
utf8_string.cpp:111:16: error: call to 'abs' is ambiguous
      else if (std::abs(index) <= signed_len) {
               ^~~~~~~~
```

Either including `cstdlib` or changing `std::abs(index)` to `std::abs((float)index)` fixed the issue with Emscripten without breaking in gcc.
Merge pull request #386 from rodneyrehm/patch-1

remove libsass.js upon `make clean`
Merge pull request #387 from rodneyrehm/patch-2

fixing `make install` (install -D) on OSX
Merge pull request #388 from rodneyrehm/patch-3

include cstdlib for std::abs() for Emscripten
Interchangeability of - and _ Fixes #200
Merge pull request #390 from wonja/interchangeable_hyphens_underscores

Interchangeability of - and _ Fixes #200
Properly handle escaped quotes. Fixes #102
Merge pull request #391 from wonja/escaped_quotes

Properly handle escaped quotes. Fixes #102
Adjacent identifiers that start with hyphens Fixes #332
Merge pull request #392 from wonja/identifiers_with_hyphens

Adjacent identifiers that start with hyphens Fixes #332
Merge remote-tracking branch 'upstream/master'