%general-entities; ]> MPFR 32 Bit CC="gcc -isystem /usr/include ${BUILD32}" \ CXX="g++ -isystem /usr/include ${BUILD32}" \ LDFLAGS="-Wl,-rpath-link,/usr/lib:/lib ${BUILD32}" \ ABI=32 ./configure \ --prefix=/usr \ --enable-cxx The header installed by GMP is architecture specific. Programs compiled as 32bit will require the header provided by the 32bit installation of GMP. The same applies for 64bit programs. Move the header so a wrapper can be put in its place later: mv -v /usr/include/gmp{,-32}.h