If your trying to install the python lxml package (specifically on Debian but this probably applies to other distros too) using either pip or easy_install and get messages like the following
Processing dependencies for scrapy
Searching for lxml
gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/python2.7 -c src/lxml/lxml.etree.c -o build/temp.linux-x86_64-2.7/src/lxml/lxml.etree.o -w
In file included from src/lxml/lxml.etree.c:239:0:
src/lxml/etree_defs.h:9:31: fatal error: libxml/xmlversion.h: No such file or directory
error: command 'gcc' failed with exit status 1
It's not obvious from the messages but chances are you need to install the following dev packages.