Error in passing CMake flags for CGAL

[JaroCamphuijsen]

Currently the readme says:

When the intention is to link with biocellion or other codes that require a relocatable library, use:
cmake -DCMAKE_BUILD_TYPE=Release -DCGAL_CXX_FLAGS="-shared -fPIC" .
(This will result in a core dump if the test code below is executed.)

However using

• Biocellion v3.1,
• gcc version 9.3.0 (Ubuntu 9.3.0-10ubuntu2)
• libcgal version 5.0.2-3 amd64

this results in an error

g++ -std=c++11 -shared  -Wl,-soname,libmodel.DP.SPAGENT.so -o libmodel.DP.SPAGENT.so interface_config.o interface_agent.o interface_mech_intrct.o interface_grid.o interface_output.o model_routine_config.o model_routine_grid.o model_routine_agent.o model_routine_mech_intrct.o model_routine_output.o interface_check.o  tetra/CMakeFiles/tetra.dir/tetra.cpp.o -lgmp
/usr/bin/ld: tetra/CMakeFiles/tetra.dir/tetra.cpp.o: relocation R_X86_64_TPOFF32 against symbol `_ZGVZN4CGAL30Triangulation_data_structure_3INS_37Triangulation_hierarchy_vertex_base_3INS_37Triangulation_vertex_base_with_info_3INS_7Point_3INS_5EpickEEES4_NS_27Triangulation_vertex_base_3IS4_NS_30Triangulation_ds_vertex_base_3INS0_INS2_IS5_S4_NS6_IS4_NS7_IvEEEEEENS_28Triangulation_ds_cell_base_3IvEENS_14Sequential_tagEEEEEEEEEEESC_SD_E21_insert_in_small_holeIN5boost9container12small_vectorINS_8internal11CC_iteratorINS_17Compact_containerINSB_ISJ_EENS_7DefaultESS_SS_EELb0EEELm32EvvEENSN_ISt4pairISU_iELm32EvvEEEENSP_INSQ_INS1_INS2_IS5_S4_NS6_IS4_NS7_ISJ_EEEEEEEESS_SS_SS_EELb0EEERKT_RKT0_E21vertex_pair_facet_map' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: tetra/CMakeFiles/tetra.dir/tetra.cpp.o: relocation R_X86_64_PC32 against symbol `_ZZN4CGAL26get_static_error_behaviourEvE16_error_behaviour' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: bad value
collect2: error: ld returned 1 exit status
make: *** [Makefile:25: libmodel.DP.SPAGENT.so] Error 1

It can be solved by changing -DCGAL_CXX_FLAGS="-shared -fPIC" to -DCMAKE_CXX_FLAGS="-shared -fPIC" in the Makefile. However this doesn't seem to be the most elegant solution, as it replaces all CXX flags added in the Makefile, rather than adding to these specifically for CGAL. However, for the use in TheCMMC/ABM-microcarriers#4, it seems to work.