Saner defaults in create_shared()

* Disable Link-Time Optimization (LTO) by default, as it greatly increases compilation time
* On UNIX-like systems, use /bin/sh for shell if SHELL environment is not set

python/treelite/contrib/clang.py

@@ -29,12 +29,12 @@ def _obj_ext():
 
 def _obj_cmd(source, options):
   obj_ext = _obj_ext()
-  return 'clang -c -O3 -o {} {} -fPIC -std=c99 -flto {}'\
+  return 'clang -c -O3 -o {} {} -fPIC -std=c99 {}'\
           .format(source + obj_ext, source + '.c', ' '.join(options))
 
 def _lib_cmd(sources, target, lib_ext, options):
   obj_ext = _obj_ext()
-  return 'clang -shared -O3 -o {} {} -std=c99 -flto {}'\
+  return 'clang -shared -O3 -o {} {} -std=c99 {}'\
           .format(target + lib_ext,
                   ' '.join([x['name'] + obj_ext for x in sources]),
                   ' '.join(options))

python/treelite/contrib/gcc.py

@@ -14,12 +14,12 @@ def _obj_ext():
 
 def _obj_cmd(source, options):
   obj_ext = _obj_ext()
-  return 'gcc -c -O3 -o {} {} -fPIC -std=c99 -flto -fopenmp {}'\
+  return 'gcc -c -O3 -o {} {} -fPIC -std=c99 -fopenmp {}'\
           .format(source + obj_ext, source + '.c', ' '.join(options))
 
 def _lib_cmd(sources, target, lib_ext, options):
   obj_ext = _obj_ext()
-  return 'gcc -shared -O3 -o {} {} -std=c99 -flto -fopenmp {}'\
+  return 'gcc -shared -O3 -o {} {} -std=c99 -fopenmp {}'\
           .format(target + lib_ext,
                   ' '.join([x['name'] + obj_ext for x in sources]),
                   ' '.join(options))

python/treelite/contrib/util.py

@@ -14,8 +14,10 @@ def _is_windows():
 def _shell():
   if _is_windows():
     return 'cmd.exe'
-  else:
+  elif 'SHELL' in os.environ:
     return os.environ['SHELL']
+  else:
+    return '/bin/sh'  # use POSIX-compliant shell if SHELL is not set
 
 def _libext():
   if _platform == 'darwin':