[VM][ENH][VMMaker] CPP directive translation for SLANG

This change set adds support for the C preprocessor to SLANG.

Two new methods are supported:
    Object>>cppDirective
    Object>>cppIfDef:inSmalltalk:comment:ifTrue:ifFalse:

Examples:

The Smalltalk expression:
    self cppDirective: 'ifdef HAS_FOO'.
    self doFooStuff.
    self cppDirective: 'endif'.
is translated to:
    # ifdef HAS_FOO
    doFooStuff();
    # endif

The Smalltalk expression:
    self cppIfDef: 'HAVE_FOO'
        inSmalltalk: [if self haveFoo]
        comment: 'some platforms do not support foo properly'
        ifTrue: [self doThingsThatWorkIfFooIsPresent]
        ifFalse: [self doSomethingElseInstead].
is translated to:
    # ifdef SOMETHING  // system supports foo
        doThingsThatWorkIfFooIsPresent();
    # else
        doSomethingElseInstead();
    # endif    // SOMETHING

Here is an example of a method that uses the C preprocessor:

primitiveUnsetEnv
    "Unset an environment variable."
    | cStringPtr keyString |
    self export: true.
    self var: 'cStringPtr' declareC: 'char *cStringPtr'.
    keyString _ interpreterProxy stackObjectValue: 0.
    cStringPtr _ self transientCStringFromString: keyString.
    self cppIfDef: 'HAVE_UNSETENV'
        inSmalltalk: [true]
        comment: 'unsetenv() is not portable'
        ifTrue: [self unsetenv: cStringPtr]
        ifFalse: [interpreterProxy primitiveFail].
    interpreterProxy pop: 1

The generated C code in the plugin is:

EXPORT(int) primitiveUnsetEnv(void) {
    int keyString;
    char *cStringPtr;

        keyString = interpreterProxy->stackObjectValue(0);
        cStringPtr = transientCStringFromString(keyString);
        # ifdef HAVE_UNSETENV   // unsetenv() is not portable
        unsetenv(cStringPtr);
        # else
        interpreterProxy->primitiveFail();
        # endif // HAVE_UNSETENV

        interpreterProxy->pop(1);
}


Dave