classpath: fix build errors with gmp support

gmp support has a hardcoded /usr/include include PATH which causes
breakage when there is a distribution gmp with development headers
installed. Fixes:
http://autobuild.buildroot.net/results/849/8497648c0b21390782eecc5b4aa91281bcf58ad6/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>