fix(mr_ckey): add C++ compatibility with GCC7

g++7 fails to initialize C-style tagged unions with the following error
message:

> sorry, unimplemented: non-trivial designated initializers not
> supported

Note that gcc7 handles this fine when the file is parsed as C, as well
as g++8 and beyond, and the problem only exists when operating in C++
mode under g++7. AL2 is still using this ancient toolchain, so add a
workaround in the case that it is used.

include/nccl_ofi_mr.h

@@ -20,6 +20,14 @@ extern "C" {
 
 #include <rdma/fi_domain.h>
 
+/* AL2's GCC7 is incapable of initializing tagged unions without these
+* constructors, but only in C++ mode.. */
+#if defined(__cplusplus) && defined(__GNUG__) && defined(__GNUC__) && __GNUC__ < 8
+#define GCC_7_CPP_COMPAT_REQUIRED 1
+#else
+#define GCC_7_CPP_COMPAT_REQUIRED 0
+#endif
+
 enum nccl_ofi_mr_ckey_type {
 	NCCL_OFI_MR_CKEY_INVALID = 0,
 	NCCL_OFI_MR_CKEY_IOVEC,
@@ -37,7 +45,18 @@ struct nccl_ofi_mr_ckey {
 #endif
 	};
 	const enum nccl_ofi_mr_ckey_type type;
+
+	/* AL2's GCC7 is incapable of initializing tagged unions without these
+	 * constructors, but only in C++ mode.. */
+#if GCC_7_CPP_COMPAT_REQUIRED
+	constexpr explicit nccl_ofi_mr_ckey(void *iov_base, size_t iov_len)
+	    : iovec{iov_base, iov_len}, type{NCCL_OFI_MR_CKEY_IOVEC} {}
+
+	constexpr explicit nccl_ofi_mr_ckey(int fd, uint64_t offset, size_t len, void *base_addr)
+	    : fi_mr_dmabuf{fd, offset, len, base_addr}, type{NCCL_OFI_MR_CKEY_DMABUF} {}
+#endif
 };
+
 typedef struct nccl_ofi_mr_ckey nccl_ofi_mr_ckey_t;
 typedef struct nccl_ofi_mr_ckey const *const nccl_ofi_mr_ckey_ref;
 
@@ -98,6 +117,10 @@ static inline uintptr_t nccl_ofi_mr_ckey_len(nccl_ofi_mr_ckey_ref ckey)
 	}
 }
 
+#if GCC_7_CPP_COMPAT_REQUIRED
+#define nccl_ofi_mr_ckey_mk_dmabuf nccl_ofi_mr_ckey
+#define nccl_ofi_mr_ckey_mk_vec    nccl_ofi_mr_ckey
+#else
 #if HAVE_DECL_FI_MR_DMABUF
 static inline nccl_ofi_mr_ckey_t nccl_ofi_mr_ckey_mk_dmabuf(int fd, uint64_t offset, size_t len, void *base_addr)
 {
@@ -125,6 +148,7 @@ static inline nccl_ofi_mr_ckey_t nccl_ofi_mr_ckey_mk_vec(void *iov_base, size_t
 		.type = NCCL_OFI_MR_CKEY_IOVEC,
 	};
 }
+#endif
 
 static inline void nccl_ofi_mr_ckey_fill_mr_attrs(nccl_ofi_mr_ckey_ref ckey, struct fi_mr_attr *attrs, uint64_t *flags)
 {