Check for a call to `mc_init()` in non-transparent executions of McMini

[maxwellpirtle]

Currently, McMini will segfault if a program makes explicit calls to McMini's wrapper functions (e.g. mc_pthread_mutex_lock()) without first calling mc_init(). We should simply bail out and recognize if/when mc_init() has been called. The only slightly tricky bit will be checking for mc_init() before each wrapper function has been called. If we can check without necessarily. We should be able to use the MC_SHARED_LIBRARY macro here

[gc00]

@maxwellpirtle , There are systems solutions to the issue that you describe.
I would recommend using "weak symbols". It will be easiest to work this out together, interactively.

Here is an example from dmtcp.h in DMTCP:

EXTERNC int dmtcp_is_enabled(DMTCP_VOID) __attribute ((weak));
#define dmtcp_is_enabled() (dmtcp_is_enabled ? dmtcp_is_enabled() : 0)

The idea is that a symbol can be declared in one unit as weak, and as a normal symbol elsewhere. In static linking, a normal symbol has priority, but a weak symbol will define the symbol to 0 (or NULL).

In dynamic linking, the system will ignore the "weak" declaration, and simply use the first symbol found in library search order. So, LD_PRELOAD helps here.

As for EXTERNC, it's a macro that expands to extern "C" only if you're in C++. It's a standard trick.