Make it possible to control STATIC_* setup variables from the environ…

[etanol]

When trying to build lxml with a statically linked libxml, the setup.py file needs to be edited or else successfully building lxml is impossible.

This PR tries to provide a way to give more control over the static build settings from environment variables.

For example, assume that libxml and libxslt have been statically built and installed under /my/salsa and we want to build lxml so that libxml and libxslt are embedded into the extension modules. With the suggested changes to the setup.py file applied, it should be possible to do it with a single pip command:

STATIC=true \
LXML_STATIC_INCLUDE_DIRS="/my/salsa/include/libxml2" \
LXML_STATIC_LIBRARY_DIRS="/my/salsa/lib" \
LXML_STATIC_BINARIES="/my/salsa/lib/libxslt.a:/my/salsa/lib/libexslt.a:/my/salsa/lib/libxml2.a" \
WITH_XML2_CONFIG="/my/salsa/bin/xml2-config" \
WITH_XSLT_CONFIG="/my/salsa/bin/xslt-config" \
      pip install --no-binary=:all: lxml

[etanol]

I thought of using | instead of , because compiler CFLAGS usually rely on the comma. For example, when passing linker flags through the front end, e.g. -Wl,-s.

[scoder]

Thanks