diff --git a/configure.ac b/configure.ac
index d1f796d3..56d0e318 100644
--- a/configure.ac
+++ b/configure.ac
@@ -154,6 +154,7 @@ dnl Checks for typedefs, structures, and compiler characteristics.
 AC_C_BIGENDIAN
 
 AC_MSG_CHECKING(for 128-bit arithmetic)
+have_ti_mode=no
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
 #include <limits.h>
 ]], [[
@@ -163,9 +164,11 @@ uint128_t x;
 return sizeof(x) / CHAR_BIT != 16U
 ]])],
 [AC_MSG_RESULT(yes)
- AC_DEFINE([HAVE_TI_MODE], [1], [gcc TI mode is available])],
+ AC_DEFINE([HAVE_TI_MODE], [1], [gcc TI mode is available])
+ have_ti_mode=yes],
 [AC_MSG_RESULT(no)])
-AC_SUBST(HAVE_TI_MODE)
+AM_CONDITIONAL([HAVE_TI_MODE], [test x$have_ti_mode = xyes])
+AC_SUBST([HAVE_TI_MODE])
 
 dnl Checks for functions and headers
 
diff --git a/src/libsodium/Makefile.am b/src/libsodium/Makefile.am
index 41353fb8..08eea70f 100644
--- a/src/libsodium/Makefile.am
+++ b/src/libsodium/Makefile.am
@@ -17,12 +17,6 @@ libsodium_la_SOURCES = \
 	crypto_generichash/blake2/ref/blake2b-ref.c \
 	crypto_generichash/blake2/ref/blake2s-ref.c \
 	crypto_generichash/blake2/ref/generichash_blake2b.c \
-	crypto_scalarmult/curve25519/donna_c64/api.h \
-	crypto_scalarmult/curve25519/donna_c64/base_curve25519_donna_c64.c \
-	crypto_scalarmult/curve25519/donna_c64/smult_curve25519_donna_c64.c \
-	crypto_scalarmult/curve25519/ref/api.h \
-	crypto_scalarmult/curve25519/ref/base_curve25519_ref.c \
-	crypto_scalarmult/curve25519/ref/smult_curve25519_ref.c \
 	crypto_hashblocks/sha256/ref/blocks_sha256.c \
 	crypto_hashblocks/sha256/ref/api.h \
 	crypto_hashblocks/sha512/ref/blocks_sha512.c \
@@ -152,6 +146,18 @@ libsodium_la_SOURCES = \
 	sodium/utils.c \
 	sodium/version.c
 
+if HAVE_TI_MODE
+libsodium_la_SOURCES += \
+	crypto_scalarmult/curve25519/donna_c64/api.h \
+	crypto_scalarmult/curve25519/donna_c64/base_curve25519_donna_c64.c \
+	crypto_scalarmult/curve25519/donna_c64/smult_curve25519_donna_c64.c
+else
+libsodium_la_SOURCES += \
+	crypto_scalarmult/curve25519/ref/api.h \
+	crypto_scalarmult/curve25519/ref/base_curve25519_ref.c \
+	crypto_scalarmult/curve25519/ref/smult_curve25519_ref.c
+endif
+
 libsodium_la_LDFLAGS = \
 	$(AM_LDFLAGS) \
 	-export-dynamic \