fix: allocating C++ compound literal on heap with Clang

fix: allocating C++ compound literal on heap with Clang

Exclude Clang from the GCC version macro check for <= 4.8 since most
versions of Clang seem to identify themselves as GCC 4.2 which in this
case forces the allocation of C++ compound literals on the heap which
is only supported starting with Clang >= 6.0.

The macro was also broken for GCC <= 4.8 in C mode, add missing
parentheses around the 'or' statement to properly distinguish between C
and C++.

Also document the minimal supported version of Clang 4.0 to build C++
probe providers.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I62eea00381b7dc5958a09b13044ad9e7f7caf2ab