package/armadillo: fix static build

By default, the library is generated as shared. Therefore, if
BR2_STATIC_LIBS is selected, let's require the creation of a
static library.

Fixes:
- https://autobuild.buildroot.org/results/fdaf2028b97f60bc01d22d3caeb9a6432d92b598

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/armadillo/armadillo.mk

@@ -30,4 +30,8 @@ ARMADILLO_CONF_OPTS += -DLAPACK_FOUND=ON
 ARMADILLO_DEPENDENCIES += lapack
 endif
 
+ifeq ($(BR2_STATIC_LIBS),y)
+ARMADILLO_CONF_OPTS += -DSTATIC_LIB=ON
+endif
+
 $(eval $(cmake-package))