build fails with Invalid TARGET_OS: darwin

[vxe]

fix sets TARGET_OS with output of uname :
https://github.com/drmeister/externals-clasp/pull/8

[ghost]

I notice this with linux too (for the testing branch). I noticed the makefile expects a capitalized 'Linux' or 'Darwin', whereas the local config examples give a lowercase 'linux' or 'darwin'. For now, capitalizing these in the makefile works for me.

[ghost]

Oops I meant capitalizing these in the config file works for me. (NOT the makefile).

[rudi]

Confirmed for OS X: changing "darwin" to "Darwin" in local.config.darwin fixes the build.