asciidoctor-mathematical and mathematical

I tried to use stem in asciidoctor-pdf. Therefore I have to use asciidoctor-mathematical and mathematical.

IThe I tried to install mathematical, but it can't be compiled because it requires libxml. On Arch Linux there is only libxml2.

"gcc -E -I/usr/include/ruby-2.3.0/x86_64-linux -I/usr/include/ruby-2.3.0/ruby/backward -I/usr/include/ruby-2.3.0 -I. -D_FORTIFY_SOURCE=2
   -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong -fPIC -I/include/libxml2 conftest.c -o conftest.i"
conftest.c:3:25: schwerwiegender Fehler: libxml/tree.h: Datei oder Verzeichnis nicht gefunden
 #include <libxml/tree.h>

Any hints?

I had the same problem on Ubuntu. I had to install the libxml2-dev package. I am not sure about Arch though.

Thank you.
After fixing that issue, I get the following warning. Looks like there is something fundamently wrong with the mathematical gem ...

current directory: /home/lmeier/.gem/ruby/2.3.0/gems/mathematical-1.6.7/ext/mathematical
make "DESTDIR="
compiling cairo_callbacks.c
compiling mathematical.c
In file included from ./mathematical.h:14:0,
                 from mathematical.c:23:
/usr/include/glib-2.0/glib/gi18n.h:26:0: Warnung: »_« redefiniert
 #define  _(String) gettext (String)
 
In file included from /usr/include/ruby-2.3.0/ruby/ruby.h:36:0,
                 from /usr/include/ruby-2.3.0/ruby.h:33,
                 from ./mathematical.h:4,
                 from mathematical.c:23:
/usr/include/ruby-2.3.0/ruby/defines.h:83:0: Anmerkung: dies ist die Stelle der vorherigen Definition
 # define _(args) args
 
mathematical.c: In Funktion »process«:
mathematical.c:104:5: Warnung: Implizite Deklaration der Funktion »mtex2MML_free_string« [-Wimplicit-function-declaration]
     mtex2MML_free_string(mathml);
     ^~~~~~~~~~~~~~~~~~~~
mathematical.c:138:77: Warnung: Übergabe des Arguments 2 von »cairo_svg_surface_create_for_stream« erzeugt Zeiger von Ganzzahl ohne Typkonvertierung [-Wint-conversion]
     surface = cairo_svg_surface_create_for_stream (cairoSvgSurfaceCallback, self, width_pt, height_pt);