Silence an unused variable warning.

Reported by Ricardo Nabinger Sanchez.
Fix inlining warning.

Add the JEMALLOC_ALWAYS_INLINE_C macro and use it for always-inlined
functions declared in .c files.  This fixes a function attribute
inconsistency for debug builds that resulted in (harmless) compiler
warnings about functions not being inlinable.

Reported by Ricardo Nabinger Sanchez.