Turn on support of redirect-malloc by default at bdwgc build

[ivmai]

It would be convenient (especially in downstream packages) to have a unified libgc build which could be used both by client requesting malloc redirection and not.
E.g. in case of a shared build, we could provide a separated proxy library, e.g. libgcredirect (which just redirect malloc to GC_redirected_malloc, etc.), and the client would use -lgcredirect if he really needs malloc redirection.

To turn off this feature, we could add new build option, e.g.: --disable-on-demand-redirect-malloc

[ivmai]

So, there could be 3 options of bdwgc build:

• no malloc redirect support (the default now)
• malloc() is exported from libgc.so (this is now if --enable-redirect-malloc)
• malloc() will be exported from libgcredirect.so, GC_redirected_malloc() will be exported from libgc.so (this will be by default after the feature is implemented)