If you have an issue on ubuntu to build monodevelop 0.18 because of an error with the libxml2.so, I have found a solution.
The error talked about gzopen64 which is not found. So I looked for it in google and find this bug report:
In fact, the bug come from a bad link with libz to check it just do
whereis libxml2 //to get where it is install.
For me it is in /usr/lib, so I do
ldd /usr/lib/libxml2.so.2
This give me this result:
linux-gate.so.1 => (0xffffe000)
libdl.so.2 => /lib/tls/i686/cmov/libdl.so.2 (0xb7e2c000)
libz.so.1 => /home/myuser/mono/lib/libz.so.1 (0xb7e17000) // this is the install folder of mono 1.2.6
libm.so.6 => /lib/tls/i686/cmov/libm.so.6 (0xb7df2000)
libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0xb7ca8000)
/lib/ld-linux.so.2 (0×80000000)
As you can see, the libz.so.1 is linked to my binary install of mono 1.2.6.
So when I check it, I see that this is a link file to libz.so.1.2.1 in the same folder.
So, I check my previous version of this lib
whereis libz
/usr/lib/libz.so.1
This file is a linked to libz.so.1.2.3.3.
Haha! Here we are, the version of ubuntu gutsy is 1.2.3.3 and the version of mono is 1.2.1.1. So I just change the link to point to the 1.2.3.3 lib. To do that you have a lot of solution.
You can copy/paste the file from /usr/lib to your mono/lib or and just change the link file libz.so.1 to link to the libz.so.1.2.3.3. (Or you can do the trick with ldconfig as said on the bug report.)
And now monodevelop build! You can enjoy all the new features 
rem: The other big issue I face of is boo build because it use nant and I do not found how to install deploy it as “make install”(copy assembly in gac and make pc file). So I use gacutil to install all dll but I miss the pc file ( package config). So, I have copy the old boo.pc from /usr/lib/pkg-config/ and change version and path to get the good boo install from my new mono 1.2.6 installation folder.
I hope this can help other people who try to have monodevelop 0.18 on ubuntu!
Merry Christmas!
