Build failure with modern GCC

[remicollet]

Using GCC 10

/usr/bin/ld: .libs/backtrace.o:(.bss+0x0): multiple definition of apm_globals'; .libs/apm.o:(.bss+0xc0): first defined here `

BTW, don't know if this project is still alive, so a simple bug report (not a PR)

[Pro-pra]

use -fcommon compile flag