build: Use config_opts instead of passing a compiler flag directly

This approach is safer as it allows the configure script to check the
availability of a compiler flag.

depends/packages/libxcb_util.mk

@@ -8,7 +8,7 @@ $(package)_dependencies=libxcb
 define $(package)_set_vars
 $(package)_config_opts = --disable-shared --disable-devel-docs --without-doxygen
 $(package)_config_opts += --disable-dependency-tracking --enable-option-checking
-$(package)_cflags += -fPIC
+$(package)_config_opts += --with-pic
 endef
 
 define $(package)_preprocess_cmds