Friendlier error when libxml2 not installed

Use distutils.ccompiler.has_function to detect whether libxml2 is
installed in setupinfo.py. This allows failing earlier with a somewhat
more helpful error message if libxml2 is not installed.

On OS X, I display a tip about `xcode-select --install`. Normally I
wouldn't put platform-specific stuff in here (i.e.: I don't advise using
apt-get on Debian/Ubuntu or yum on Fedora/RedHat/CentOS) because it's a
pain to enumerate all the special cases and usually most people on Linux
platforms know their platform well enough to know how to install
packages. I made an exception for OS X and `xcode-select --install`,
because I am pretty savvy with Linux and OS X and getting things to
build, but the `xcode-select --install` thing was not obvious to me at
all. At one point, I had concluded that libxml2 must've been removed
from OS X and I would have to `brew install` it and specify environment
variables to make the compiler find it, because it's a keg-only formula,
but then I found a post on Stack Overflow, which illustrated that
`xcode-select --install` might be all it takes to get Xcode to put the
files in the proper places so that command-line tools can find them.
Since this was not obvious and a good tip, I decided to add it here.

1 files changed, 26 insertions, 2 deletions

diff --git a/setupinfo.py b/setupinfo.py
index c6c5ae1e..1ac6f3ba 100644
--- a/setupinfo.py
+++ b/setupinfo.py
@@ -1,6 +1,6 @@
 import sys, os, os.path
 from distutils.core import Extension
-from distutils.errors import DistutilsOptionError
+from distutils.errors import CompileError, DistutilsOptionError
 from versioninfo import get_base_dir
 
 try:
@@ -200,9 +200,33 @@ def find_dependencies(module):
 def extra_setup_args():
     result = {}
     if CYTHON_INSTALLED:
-        result['cmdclass'] = {'build_ext': build_pyx}
+        class CheckLibxml2BuildExt(build_pyx):
+            """Subclass to check whether libxml2 is available"""
+
+            def run(self):
+                try:
+                    build_pyx.run(self)
+                except CompileError as e:
+                    print('Compile failed: %s' % e)
+                    if not has_libxml2():
+                        sys.stderr.write('*********************************************************************************\n')
+                        sys.stderr.write('Could not find function xmlCheckVersion in library libxml2. Is libxml2 installed?\n')
+                        if sys.platform in ('darwin',):
+                            sys.stderr.write('Perhaps try: xcode-select --install\n')
+                        sys.stderr.write('*********************************************************************************\n')
+                        raise
+
+        result['cmdclass'] = {'build_ext': CheckLibxml2BuildExt}
     return result
 
+def has_libxml2():
+    from distutils import ccompiler
+    compiler = ccompiler.new_compiler()
+    return compiler.has_function(
+        'xmlXPathInit',
+        include_dirs=['/usr/include/libxml2'], includes=['libxml/xpath.h'],
+        libraries=['xml2'])
+
 def libraries():
     if sys.platform in ('win32',):
         libs = ['libxslt', 'libexslt', 'libxml2', 'iconv']