package/brotli: properly pass TARGET_CFLAGS in Microblaze workaround

In commit 39dde1ffb6a7111c2fa2b340c940e4d3e685aa29 ("package/brotli:
work around gcc bug 68485"), a mistake was made: when
BR2_TOOLCHAIN_HAS_GCC_BUG_68485=y, we're only passing -O0 as CFLAGS,
completely ignoring TARGET_CFLAGS, which is not good.

This commit fixes that by introducing a BROTLI_CFLAGS variable,
initially defined to TARGET_CFLAGS, and to which we add -O0 when
BR2_TOOLCHAIN_HAS_GCC_BUG_68485=y.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
[Thomas: rework commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

(cherry picked from commit 657ffee375f4751a760b28795c7064e89b882147)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/brotli/brotli.mk

@@ -14,8 +14,12 @@ BROTLI_CONF_OPTS = \
 	-DBROTLI_DISABLE_TESTS=ON \
 	-DBROTLI_BUNDLED_MODE=OFF
 
+BROTLI_CFLAGS = $(TARGET_CFLAGS)
+
 ifeq ($(BR2_TOOLCHAIN_HAS_GCC_BUG_68485),y)
-BROTLI_CONF_OPTS += -DCMAKE_C_FLAGS="-O0"
+BROTLI_CFLAGS += -O0
 endif
 
+BROTLI_CONF_OPTS += -DCMAKE_C_FLAGS="$(BROTLI_CFLAGS)"
+
 $(eval $(cmake-package))