package/gmp: enable C++ support when BR2_INSTALL_LIBSTDCPP=y

gmp has optional C++ support, which is disabled by default. Let's
enabled it conditionally depending on the BR2_INSTALL_LIBSTDCPP
option.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/gmp/gmp.mk

@@ -19,5 +19,11 @@ ifeq ($(BR2_m68k_cf)$(BR2_MIPS_CPU_MIPS32R6)$(BR2_MIPS_CPU_MIPS64R6)$(BR2_ARM_CP
 GMP_CONF_OPTS += --disable-assembly
 endif
 
+ifeq ($(BR2_INSTALL_LIBSTDCPP),y)
+GMP_CONF_OPTS += --enable-cxx
+else
+GMP_CONF_OPTS += --disable-cxx
+endif
+
 $(eval $(autotools-package))
 $(eval $(host-autotools-package))