I downloaded the package from here:
http://packages.ubuntu.com/hardy/i386/libmpfr1ldbl/download and installed it. Then it worked!

BEFORE DOWNLOADING:
mstevens@helium:~/code2/antipasto_arduino/antipasto_arduino/build/linux/work/hardware/tools/avr/libexec/gcc/avr/4.3.3$ ldd cc1
linux-gate.so.1 => (0x00116000)
libmpfr.so.1 => not found
libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0x001d7000)
/lib/ld-linux.so.2 (0x00733000)
mstevens@helium:~/code2/antipasto_arduino/antipasto_arduino/build/linux/work/hardware/tools/avr/libexec/gcc/avr/4.3.3$ ldd cc1plus
linux-gate.so.1 => (0x008ec000)
libmpfr.so.1 => not found
libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0x00c9e000)
/lib/ld-linux.so.2 (0x007c5000)
AFTER DOWNLOADING:
mstevens@helium:/usr/lib$ ls *mpfr*
libmpfr.so.1 libmpfr.so.1.1.1 libmpfr.so.4 libmpfr.so.4.0.1
mstevens@helium:~/code2/antipasto_arduino/antipasto_arduino/build/linux/work/hardware/tools/avr/libexec/gcc/avr/4.3.3$ ldd cc1plus
linux-gate.so.1 => (0x004c4000)
libmpfr.so.1 => /usr/lib/libmpfr.so.1 (0x0080f000)
libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0x00e63000)
libgmp.so.3 => /usr/lib/libgmp.so.3 (0x005ab000)
/lib/ld-linux.so.2 (0x007e9000)
FYI