每次我尝试安装 lxml 包时，最后都会出现以下错误。
llvm-gcc-4.2 -fno-strict-aliasing -fno-common -dynamic -g -Os -pipe -fno-common -fno-strict-
aliasing -fwrapv -mno-fused-madd -DENABLE_DTRACE -DMACOSX -DNDEBUG -Wall -Wstrict-prototypes -Wshorten-64-to-32 -DNDEBUG -g -fwrapv -Os -Wall -Wstrict-prototypes -DENABLE_DTRACE -arch i386 -arch x86_64 -pipe -I/usr/include/libxml2 -I/private/var/folders/43/m1qv9zf53q19sqh6h9kg9pz80000gn/T/pip_build_sanjeevkumar/lxml/src/lxml/includes -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c src/lxml/lxml.etree.c -o build/temp.macosx-10.7-intel-2.7/src/lxml/lxml.etree.o -flat_namespace
unable to execute llvm-gcc-4.2: No such file or directory
error: command 'llvm-gcc-4.2' failed with exit status 1