Makefile: move definitions of CC, CXX, JAVACC for Cygwin

The CC, CXX and JAVACC makefile variables are all redefined in the
"compiler flags" section of the makefile, which makes them easy to miss.
Define them in the "compilers" section instead.

prism/Makefile

@@ -127,11 +127,24 @@ JAVA_DIR_BACKUP = $(shell \
 # Compilers etc. #
 ##################
 
-export CC = gcc
-export CXX = g++
-export LD = $(CXX)
-export JAVAC = javac
-export JAVACC = javacc
+ifeq ($(OSTYPE),cygwin)
+	ifeq ($(ARCH),x86_64)
+		CC = /usr/bin/x86_64-w64-mingw32-gcc
+		CXX = /usr/bin/x86_64-w64-mingw32-g++
+	else
+		CC = /usr/bin/i686-w64-mingw32-gcc
+		CXX = /usr/bin/i686-w64-mingw32-g++
+	endif
+	JAVACC = javacc.bat
+else
+	CC = gcc
+	CXX = g++
+	JAVACC = javacc
+endif
+LD = $(CXX)
+JAVAC = javac
+
+export CC CXX LD JAVAC JAVACC
 
 ##############
 # Flags etc. #
@@ -209,8 +222,6 @@ endif
 # Cygwin
 ifeq ($(OSTYPE),cygwin)
 	ifeq ($(ARCH),x86_64)
-		CC = /usr/bin/x86_64-w64-mingw32-gcc
-		CXX = /usr/bin/x86_64-w64-mingw32-g++
 		CUDD_XCFLAGS = -malign-double -DHAVE_IEEE_754 -DHAVE_GETRLIMIT=0 -DRLIMIT_DATA_DEFAULT=268435456 -DHAVE_SYS_RESOURCE_H=0 -DHAVE_SYS_WAIT_H=0 -DSIZEOF_VOID_P=8 -DSIZEOF_LONG=4 $(DEBUG) $(WARNINGS) -static-libgcc -static-libstdc++
 		CFLAGS = $(CUDD_XCFLAGS) $(OPTIMISE)
 		# need -DWIN32 for lpsolve (WIN32 is only defined by mingw/gcc without --std=c++11)
@@ -218,8 +229,6 @@ ifeq ($(OSTYPE),cygwin)
 		LDFLAGS = $(CUDD_XCFLAGS) $(OPTIMISE) -Wl,--add-stdcall-alias -Wl,-Bstatic,--whole-archive -lpthread -Wl,-Bdynamic,--no-whole-archive
 		BINDISTSUFFIX = win64
 	else
-		CC = /usr/bin/i686-w64-mingw32-gcc
-		CXX = /usr/bin/i686-w64-mingw32-g++
 		CUDD_XCFLAGS = -march=i686 -malign-double -DHAVE_IEEE_754 -DHAVE_GETRLIMIT=0 -DRLIMIT_DATA_DEFAULT=268435456 -DHAVE_SYS_RESOURCE_H=0 -DHAVE_SYS_WAIT_H=0 $(DEBUG) $(WARNINGS) -static-libgcc -static-libstdc++
 		CFLAGS = $(CUDD_XCFLAGS) $(OPTIMISE)
 		# need -DWIN32 for lpsolve (WIN32 is only defined by mingw/gcc without --std=c++11)
@@ -229,7 +238,6 @@ ifeq ($(OSTYPE),cygwin)
 	endif
 	BIN_TARGETS=prism.cygwin xprism.linux prism.bat.win xprism.bat.win
 	JFLAGS = -encoding UTF8
-	JAVACC = javacc.bat
 	SHARED = -shared
 	#SHARED = -G
 	EXE = .exe