package/swipl: fix build without C++ compiler

Fixes http://autobuild.buildroot.net/results/b24a96b999676990995a466686fe1eacf6cddfef

Some of the optional packages in swipl are written in C++, so CMake thinks a
C++ compiler is needed even though we build with -DSWIPL_PACKAGES=OFF since
the bump to 9.2.6 in commit 33d45b9c672df0 ("package/swipl: bump version to
9.2.6").

Add -DCMAKE_CXX_COMPILER=true to satisfy this check even in setups without
C++:

-- Check for working CXX compiler: /usr/bin/true
-- Check for working CXX compiler: /usr/bin/true - works

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/swipl/swipl.mk

@@ -36,7 +36,8 @@ SWIPL_CONF_OPTS = \
 	-DQSORT_R_GNU=1 \
 	-DSWIPL_NATIVE_FRIEND=$(HOST_SWIPL_SRCDIR) \
 	-DSWIPL_PACKAGES=OFF \
-	-DUSE_TCMALLOC=OFF
+	-DUSE_TCMALLOC=OFF \
+	-DCMAKE_CXX_COMPILER=true
 
 ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
 SWIPL_CONF_OPTS += -DCMAKE_EXE_LINKER_FLAGS=-latomic