Implement passing GYP_DEFINES as an environment variable

This closes #229

Author: ignisf

CHANGELOG.md

@@ -3,6 +3,7 @@
 * Fix architecture detection for armv7 #226
 * Update upstream v8 version to 5.4.500.31
 * Dramatically reduce the binary size on macOS (thanks @aviat)
+* Allow passing of GYP_DEFINES as an environment variable
 
 ### v5.3.332.38.2, v5.3.332.38.3 - 2016-11-21
 

ext/libv8/builder.rb

@@ -35,6 +35,9 @@ def gyp_defines(*defines)
       # Pass clang flag to GYP in order to work around GCC compilation failures
       defines << "clang=#{@compiler.is_a?(Compiler::Clang) ? '1' : '0'}"
 
+      # Add contents of the GYP_DEFINES environment variable if present
+      defines << ENV['GYP_DEFINES'] unless ENV['GYP_DEFINES'].nil?
+
       "GYP_DEFINES=\"#{defines.join ' '}\""
     end