open-plc-utils: fix build on !uclibc toolchains

The open-plc-utils source code has a condition for uClibc toolchains
when calling vfprintf(). But the non-uClibc case simply doesn't
build. And the recommended solution at
https://github.com/qca/open-plc-utils/issues/36 is to pass
-D__UCLIBC__ unconditionally.

Fixes:

  http://autobuild.buildroot.org/results/4fd/4fdc56f5dec6c8773086a4661eff1e4ce6ef660e/

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

package/open-plc-utils/open-plc-utils.mk

@@ -11,8 +11,12 @@ OPEN_PLC_UTILS_LICENSE_FILES = LICENSE
 
 # We unfortunately can't use TARGET_CONFIGURE_OPTS, as it would
 # override the package CFLAGS, LDFLAGS and CXXFLAGS definitions.
+#
+# Yes, we're passing __UCLIBC__ in EXTRA_CFLAGS, as it fixes a build
+# issue for non-uClibc toolchains. It is the very crappy solution
+# suggested at https://github.com/qca/open-plc-utils/issues/36.
 define OPEN_PLC_UTILS_BUILD_CMDS
-	$(TARGET_MAKE_ENV) $(MAKE) CROSS=$(TARGET_CROSS) -C $(@D)
+	$(TARGET_MAKE_ENV) $(MAKE) CROSS=$(TARGET_CROSS) EXTRA_CFLAGS="-D__UCLIBC__" -C $(@D)
 endef
 
 define OPEN_PLC_UTILS_INSTALL_TARGET_CMDS