meson: Fix build failure with gcc10 on mingw

We were explicitly setting c_args and c_link_args to [], which
overrode the values inherited from the env via CFLAGS and LDFLAGS.

Also add a comment for future reference.

Fixes https://gitlab.freedesktop.org/fontconfig/fontconfig/-/issues/263
diff --git a/.gitlab-ci/linux-mingw-w64-64bit.txt b/.gitlab-ci/linux-mingw-w64-64bit.txt
index 0535654..fd4a640 100644
--- a/.gitlab-ci/linux-mingw-w64-64bit.txt
+++ b/.gitlab-ci/linux-mingw-w64-64bit.txt
@@ -5,8 +5,9 @@
 endian = 'little'
 
 [properties]
-c_args = []
-c_link_args = []
+# Will be picked up from env vars if not defined here
+#c_args = []
+#c_link_args = []
 
 [binaries]
 c = 'x86_64-w64-mingw32-gcc'