Use a clean environment when building harfbuzz

Prevent CMake from picking up the parent project CFLAGS.

Fixes: emscripten-core/emsdk#796

Author: albertvaka

tools/ports/harfbuzz.py

@@ -7,6 +7,8 @@
 import shutil
 import logging
 
+from tools import system_libs
+
 TAG = '1.7.5'
 HASH = 'c2c13fc97bb74f0f13092b07804f7087e948bce49793f48b62c2c24a5792523acc0002840bebf21829172bb2e7c3df9f9625250aec6c786a55489667dd04d6a0'
 
@@ -60,7 +62,7 @@ def create(final):
       cmake_cmd += ['-DCMAKE_CXX_FLAGS="{}"'.format(' '.join(extra_cflags))]
       cmake_cmd += ['-DCMAKE_C_FLAGS="{}"'.format(' '.join(extra_cflags))]
 
-    shared.run_process(cmake_cmd)
+    shared.run_process(cmake_cmd, env=system_libs.clean_env())
     shared.run_process(['cmake', '--build', build_path, '--target', 'install'])
 
     ports.install_header_dir(os.path.join(build_path, 'include', 'harfbuzz'))