open-plc-utils: fix build with static musl

Pass TARGET_LDFLAGS to EXTRA_LDFLAGS to fix following issue:
/home/buildroot/autobuild/run/instance-3/output/build/host-gcc-final-7.3.0/build/arm-buildroot-linux-musleabihf/libgcc/../../../libgcc/config/arm/lib1funcs.S:1545: undefined reference to `raise'

Also pass TARGET_CFLAGS to EXTRA_CFLAGS and TARGET_CXXFLAGS to
EXTRA_CXXFLAGS and move all these variables to
OPEN_PLC_UTILS_MAKE_OPTS for readability

Fixes:
 - http://autobuild.buildroot.org/results/67bc5e7ac8ae1c49c035b022a394d2f746705cf2

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

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

@@ -15,8 +15,14 @@ OPEN_PLC_UTILS_LICENSE_FILES = LICENSE
 # 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.
+OPEN_PLC_UTILS_MAKE_OPTS = \
+	EXTRA_CFLAGS="$(TARGET_CFLAGS) -D__UCLIBC__" \
+	EXTRA_CXXFLAGS="$(TARGET_CXXFLAGS)" \
+	EXTRA_LDFLAGS="$(TARGET_LDFLAGS)"
+
 define OPEN_PLC_UTILS_BUILD_CMDS
-	$(TARGET_MAKE_ENV) $(MAKE) CROSS=$(TARGET_CROSS) EXTRA_CFLAGS="-D__UCLIBC__" -C $(@D)
+	$(TARGET_MAKE_ENV) $(MAKE) CROSS=$(TARGET_CROSS) \
+		$(OPEN_PLC_UTILS_MAKE_OPTS) -C $(@D)
 endef
 
 define OPEN_PLC_UTILS_INSTALL_TARGET_CMDS