Update documentation for building cbmc on windows.

Update makefiles to use reasonable default compiler for cygwin build.
Allow alternative downloader selection from make command line.

Author: andreast271

src/Makefile

@@ -63,11 +63,13 @@ clean: $(patsubst %, %_clean, $(DIRS))
 $(patsubst %, %_clean, $(DIRS)):
 	$(MAKE) $(MAKEARGS) -C $(patsubst %_clean, %, $@) clean ; \
 
-# minisat 2 download, for your convenience
+# minisat2 and glucose download, for your convenience
+
+DOWNLOADER = lwp-download
 
 minisat2-download:
 	@echo "Downloading Minisat 2.2.1"
-	@lwp-download http://ftp.debian.org/debian/pool/main/m/minisat2/minisat2_2.2.1.orig.tar.gz
+	@$(DOWNLOADER) http://ftp.debian.org/debian/pool/main/m/minisat2/minisat2_2.2.1.orig.tar.gz
 	@tar xfz minisat2_2.2.1.orig.tar.gz
 	@rm -Rf ../minisat-2.2.1
 	@mv minisat2-2.2.1 ../minisat-2.2.1
@@ -76,7 +78,7 @@ minisat2-download:
 
 glucose-download:
 	@echo "Downloading glucose-syrup"
-	@lwp-download http://www.labri.fr/perso/lsimon/downloads/softwares/glucose-syrup.tgz
+	@$(DOWNLOADER) http://www.labri.fr/perso/lsimon/downloads/softwares/glucose-syrup.tgz
 	@tar xfz glucose-syrup.tgz
 	@rm -Rf ../glucose-syrup
 	@mv glucose-syrup ../

src/common

@@ -101,17 +101,19 @@ else ifeq ($(BUILD_ENV_),Cygwin)
   CXXFLAGS ?= -Wall -O2
   CP_CFLAGS = -MMD -MP
   CP_CXXFLAGS += -MMD -MP -std=c++11 -U__STRICT_ANSI__
+  # Cygwin-g++ has problems with statically linking exception code.
+  # If linking fails, remove -static.
   LINKFLAGS = -static -std=c++11
   LINKLIB = ar rcT $@ $^
-  LINKBIN = $(CXX) $(LINKFLAGS) -o $@ -Wl,--start-group $^ -Wl,--end-group $(LIBS) -static
+  LINKBIN = $(CXX) $(LINKFLAGS) -o $@ -Wl,--start-group $^ -Wl,--end-group $(LIBS)
   LINKNATIVE = $(HOSTCXX) -std=c++11 -o $@ $^ -static
 ifeq ($(origin CC),default)
   #CC = gcc
-  CC = i686-w64-mingw32-gcc
+  CC = x86_64-w64-mingw32-gcc
 endif
 ifeq ($(origin CXX),default)
   #CXX    = g++
-  CXX = i686-w64-mingw32-g++
+  CXX = x86_64-w64-mingw32-g++
 endif
 ifeq ($(origin YACC),default)
   YACC   = bison -y