gyp: inherit parent for *.host

Gyp defaults to gcc/g++ if CC.host/CXX.host is unset. This is not
suitable for environments that only uses the clang toolchain.

Since we already assume that the user will provide clang/clang++
through CC/CXX, lean against it (then drop to gcc/g++).

Also apply the same logic for link/ar for consistency although
it doesn't affect us.

PR-URL: nodejs/node#6173
Fixes: nodejs/node#6152
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

tools/gyp/pylib/gyp/generator/make.py

@@ -2074,10 +2074,10 @@ def CalculateMakefilePath(build_file, base_name):
     'AR.target':   GetEnvironFallback(('AR_target', 'AR'), '$(AR)'),
     'CXX.target':  GetEnvironFallback(('CXX_target', 'CXX'), '$(CXX)'),
     'LINK.target': GetEnvironFallback(('LINK_target', 'LINK'), '$(LINK)'),
-    'CC.host':     GetEnvironFallback(('CC_host',), 'gcc'),
-    'AR.host':     GetEnvironFallback(('AR_host',), 'ar'),
-    'CXX.host':    GetEnvironFallback(('CXX_host',), 'g++'),
-    'LINK.host':   GetEnvironFallback(('LINK_host',), '$(CXX.host)'),
+    'CC.host':     GetEnvironFallback(('CC_host', 'CC'), 'gcc'),
+    'AR.host':     GetEnvironFallback(('AR_host', 'AR'), 'ar'),
+    'CXX.host':    GetEnvironFallback(('CXX_host', 'CXX'), 'g++'),
+    'LINK.host':   GetEnvironFallback(('LINK_host', 'LINK'), '$(CXX.host)'),
   })
 
   build_file, _, _ = gyp.common.ParseQualifiedTarget(target_list[0])