libcap: fix build failure

On a x86-64 host, libcap tried to install things in
$(STAGING_DIR)/lib64 and $(TARGET_DIR)/lib64. Therefore, pass lib= and
prefix=, as required by the strange build system used by libcap.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libcap/libcap.mk

@@ -8,7 +8,7 @@ define LIBCAP_BUILD_CMDS
 endef
 
 define LIBCAP_INSTALL_STAGING_CMDS
- $(TARGET_MAKE_ENV) $(MAKE) -C $(@D) LIBATTR=no DESTDIR=$(STAGING_DIR) install
+ $(TARGET_MAKE_ENV) $(MAKE) -C $(@D) LIBATTR=no DESTDIR=$(STAGING_DIR) prefix=/usr lib=lib install
 endef
 
 define LIBCAP_INSTALL_TARGET_CMDS
@@ -20,7 +20,7 @@ define HOST_LIBCAP_BUILD_CMDS
 endef
 
 define HOST_LIBCAP_INSTALL_CMDS
- $(HOST_MAKE_ENV) $(MAKE) -C $(@D) LIBATTR=no DESTDIR=$(HOST_DIR) install
+ $(HOST_MAKE_ENV) $(MAKE) -C $(@D) LIBATTR=no DESTDIR=$(HOST_DIR) prefix=/usr lib=lib install
 endef
 
 $(eval $(call GENTARGETS,package,libcap))