I've been installing software packages as a part of developing for the AVR micro-controllers. In the process, I successfully built and installed gmp-4.3.2
As a next step, I went on to build mpfr.2.4.2

My command was like:
prasan@ubuntu:~/Downloads/mpfr-2.4.2$ ./configure --prefix=/usr/local/AVR

But I got an error as follows:

......
checking for gmp.h... no
configure: error: gmp.h can't be found, or is unusable.

I even checked in the /usr/local/AVR/include directory for a gmp.h file and it was there.
Help please!