There are two build errors. The first is:
/bin/sh ../libtool --tag=CXX --mode=compile g++ -DHAVE_CONFIG_H -I. -I../include -I../include -I./include -I. -I../libbroad -I../libmoto -g -O2 -MT C-api.lo -MD -MP -MF .deps/C-api.Tpo -c -o C-api.lo C-api.cpp
g++ -DHAVE_CONFIG_H -I. -I../include -I../include -I./include -I. -I../libbroad -I../libmoto -g -O2 -MT C-api.lo -MD -MP -MF .deps/C-api.Tpo -c C-api.cpp -fPIC -DPIC -o .libs/C-api.o
C-api.cpp: In function 'void dtVertexIndices(DtPolyType, DT_Count, const DT_Index*)':
C-api.cpp:156:18: error: expected type-specifier
C-api.cpp:156:18: error: cannot convert 'int*' to 'const Polytope*' in assignment
C-api.cpp:156:18: error: expected ';'
make[1]: *** [C-api.lo] Error 1
make[1]: Leaving directory `/home/tlow/src/freesolid/FreeSOLID-2.1.1/libsolid'
make: *** [all-recursive] Error 1
After fixing that one, a second error occurs:
make[1]: Entering directory `/home/tlow/src/freesolid/FreeSOLID-2.1.1/libsolid'
/bin/sh ../libtool --tag=CXX --mode=link g++ -g -O2 -no-undefined -version-info 2:1:2 -release 2.1.1 -o libFreeSOLID.la -rpath /usr/local/lib Transform.lo Convex.lo Box.lo Cone.lo Cylinder.lo Sphere.lo Simplex.lo Polygon.lo Polyhedron.lo Complex.lo BBoxTree.lo Object.lo Response.lo RespTable.lo C-api.lo BP_C-api.lo BP_Endpoint.lo BP_Proxy.lo BP_Scene.lo GEN_random.lo MT_Matrix3x3.lo MT_Point2.lo MT_Point3.lo MT_Quaternion.lo MT_Transform.lo MT_Vector2.lo MT_Vector3.lo MT_Vector4.lo -lqhull -lm
g++ -shared /usr/lib/gcc/i486-linux-gnu/4.7/../../../i386-linux-gnu/crti.o /usr/lib/gcc/i486-linux-gnu/4.7/crtbeginS.o .libs/Transform.o .libs/Convex.o .libs/Box.o .libs/Cone.o .libs/Cylinder.o .libs/Sphere.o .libs/Simplex.o .libs/Polygon.o .libs/Polyhedron.o .libs/Complex.o .libs/BBoxTree.o .libs/Object.o .libs/Response.o .libs/RespTable.o .libs/C-api.o .libs/BP_C-api.o .libs/BP_Endpoint.o .libs/BP_Proxy.o .libs/BP_Scene.o .libs/GEN_random.o .libs/MT_Matrix3x3.o .libs/MT_Point2.o .libs/MT_Point3.o .libs/MT_Quaternion.o .libs/MT_Transform.o .libs/MT_Vector2.o .libs/MT_Vector3.o .libs/MT_Vector4.o -lqhull -L/usr/lib/gcc/i486-linux-gnu/4.7 -L/usr/lib/gcc/i486-linux-gnu/4.7/../../../i386-linux-gnu -L/usr/lib/gcc/i486-linux-gnu/4.7/../../../../lib -L/lib/i386-linux-gnu -L/lib/../lib -L/usr/lib/i386-linux-gnu -L/usr/lib/../lib -L/usr/lib/gcc/i486-linux-gnu/4.7/../../.. -lstdc++ -lm -lc -lgcc_s /usr/lib/gcc/i486-linux-gnu/4.7/crtendS.o /usr/lib/gcc/i486-linux-gnu/4.7/../../../i386-linux-gnu/crtn.o -o .libs/libFreeSOLID-2.1.1.so.0.2.1
/usr/lib/gcc/i486-linux-gnu/4.7/../../../i386-linux-gnu/crti.o: In function `_init':
(.init+0x0): multiple definition of `_init'
/usr/lib/gcc/i486-linux-gnu/4.7/../../../i386-linux-gnu/crti.o:(.init+0x0): first defined here
/usr/lib/gcc/i486-linux-gnu/4.7/../../../i386-linux-gnu/crti.o: In function `_fini':
(.fini+0x0): multiple definition of `_fini'
/usr/lib/gcc/i486-linux-gnu/4.7/../../../i386-linux-gnu/crti.o:(.fini+0x0): first defined here
/usr/lib/gcc/i486-linux-gnu/4.7/crtbeginS.o:(.data.rel+0x0): multiple definition of `__dso_handle'
/usr/lib/gcc/i486-linux-gnu/4.7/crtbeginS.o:(.data.rel+0x0): first defined here
/usr/lib/gcc/i486-linux-gnu/4.7/crtendS.o:(.tm_clone_table+0x0): multiple definition of `__TMC_END__'
/usr/lib/gcc/i486-linux-gnu/4.7/crtendS.o:(.tm_clone_table+0x0): first defined here
collect2: error: ld returned 1 exit status
make[1]: *** [libFreeSOLID.la] Error 1
make[1]: Leaving directory `/home/tlow/src/freesolid/FreeSOLID-2.1.1/libsolid'
make: *** [all-recursive] Error 1
Attcahed patch fixes both errors but the fix for for the second error is likely not the correct way to solve the problem.
Fixes two build errors on Debian unstable