Exclude file that calls system() from build

system() is unavailable on iOS. Fortunately the caller function is not
used elsewhere so it's possible to simply remove source file from the
set of files to compile

src/lib/third_party/numerics/CMakeLists.txt

@@ -22,6 +22,10 @@ aux_source_directories(SOURCES "Source Files\\SUPERLU" SUPERLU)
 aux_source_directories(SOURCES "Source Files\\ARPACK" ARPACK)
 aux_source_directories(SOURCES "Source Files\\ARPACK_UTIL" ARPACK_UTIL)
 
+# This file uses system() which is not available on iOS. The caller function is not used elsewhere
+# so just remove source file from the set of files to compile
+list(REMOVE_ITEM SOURCES "LIBF2C/system_.c")
+
 add_library(geogram_num_3rdparty ${SOURCES})
 
 if(UNIX)