Export CC on Windows

Considering we require this compiler, we should set a good
default for all sub-Makefiles.

plugins/c_src.mk

@@ -30,6 +30,7 @@ ifeq ($(PLATFORM),msys2)
 # We hardcode the compiler used on MSYS2. The default CC=cc does
 # not produce working code. The "gcc" MSYS2 package also doesn't.
 	CC = /mingw64/bin/gcc
+	export CC
 	CFLAGS ?= -O3 -std=c99 -finline-functions -Wall -Wmissing-prototypes
 	CXXFLAGS ?= -O3 -finline-functions -Wall
 else ifeq ($(PLATFORM),darwin)