Fix Cygwin build and document needed packages

Makefile

@@ -175,6 +175,12 @@ LD = gcc-4.8
 CXXFLAGS += -std=c++11 -Os
 ABCMKARGS += ARCHFLAGS="-DABC_USE_STDINT_H"
 
+else ifeq ($(CONFIG),cygwin)
+CXX = gcc
+LD = gcc
+CXXFLAGS += -std=gnu++11 -Os
+ABCMKARGS += ARCHFLAGS="-DABC_USE_STDINT_H"
+
 else ifeq ($(CONFIG),emcc)
 CXX = emcc
 LD = emcc
@@ -729,6 +735,9 @@ config-msys2: clean
 config-msys2-64: clean
 	echo 'CONFIG := msys2-64' > Makefile.conf
 
+config-cygwin: clean
+	echo 'CONFIG := cygwin' > Makefile.conf
+
 config-gcov: clean
 	echo 'CONFIG := gcc' > Makefile.conf
 	echo 'ENABLE_GCOV := 1' >> Makefile.conf

README.md

@@ -69,6 +69,10 @@ On FreeBSD use the following command to install all prerequisites:
 On FreeBSD system use gmake instead of make. To run tests use:
     % MAKE=gmake CC=cc gmake test
 
+For Cygwin use the following command to install all prerequisites, or select these additional packages:
+
+	setup-x86_64.exe -q --packages=bison,flex,gcc-core,gcc-g++,git,libffi-devel,libreadline-devel,make,pkg-config,python3,tcl-devel
+
 There are also pre-compiled Yosys binary packages for Ubuntu and Win32 as well
 as a source distribution for Visual Studio. Visit the Yosys download page for
 more information: http://www.clifford.at/yosys/download.html

kernel/yosys.cc

@@ -166,7 +166,7 @@ std::string vstringf(const char *fmt, va_list ap)
 	std::string string;
 	char *str = NULL;
 
-#ifdef _WIN32
+#if defined(_WIN32 )|| defined(__CYGWIN__) 
 	int sz = 64, rc;
 	while (1) {
 		va_list apc;