Eu não sei porque você é incapaz de localizar o pacote, mas você pode baixá-lo de packages.ubuntu .com e instale-o manualmente. Vá até o local do arquivo baixado e digite:
% bl0ck_qu0te%E eu preciso disso para um servidor que não morra de fome
apt-get update && apt-get install libcurl4-gnutls-dev:i386
Ign http://archive.ubuntu.com trusty InRelease
Ign http://archive.ubuntu.com trusty-updates InRelease
Ign http://archive.ubuntu.com trusty-security InRelease
Hit http://archive.ubuntu.com trusty Release.gpg
Hit http://archive.ubuntu.com trusty-updates Release.gpg
Hit http://archive.ubuntu.com trusty-security Release.gpg
Hit http://archive.ubuntu.com trusty Release
Hit http://archive.ubuntu.com trusty-updates Release
Hit http://archive.ubuntu.com trusty-security Release
Hit http://archive.ubuntu.com trusty/main Sources
Hit http://archive.ubuntu.com trusty/restricted Sources
Hit http://archive.ubuntu.com trusty/universe Sources
Hit http://archive.ubuntu.com trusty/main amd64 Packages
Hit http://archive.ubuntu.com trusty/restricted amd64 Packages
Hit http://archive.ubuntu.com trusty/universe amd64 Packages
Hit http://archive.ubuntu.com trusty-updates/main Sources
Hit http://archive.ubuntu.com trusty-updates/restricted Sources
Hit http://archive.ubuntu.com trusty-updates/universe Sources
Hit http://archive.ubuntu.com trusty-updates/main amd64 Packages
Hit http://archive.ubuntu.com trusty-updates/restricted amd64 Packages
Hit http://archive.ubuntu.com trusty-updates/universe amd64 Packages
Hit http://archive.ubuntu.com trusty-security/main Sources
Hit http://archive.ubuntu.com trusty-security/restricted Sources
Hit http://archive.ubuntu.com trusty-security/universe Sources
Hit http://archive.ubuntu.com trusty-security/main amd64 Packages
Hit http://archive.ubuntu.com trusty-security/restricted amd64 Packages
Hit http://archive.ubuntu.com trusty-security/universe amd64 Packages
Reading package lists... Done
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package libcurl4-gnutls-dev
Eu não sei porque você é incapaz de localizar o pacote, mas você pode baixá-lo de packages.ubuntu .com e instale-o manualmente. Vá até o local do arquivo baixado e digite:
% bl0ck_qu0te%Eu tive que instalar a arquitetura i386.
% bl0ck_qu0te%atualize o apt e tente instalar novamente
% bl0ck_qu0te%APM