ENH: Auto-import symbols for cygwin executables

This enables the --enable-auto-import linker flag on Cygwin when linking
executables.  It works with the old gcc 3.x compiler and is necessary
for the new gcc 4.x compiler.  See issue #9071.

Modules/Platform/CYGWIN.cmake

 SET(WIN32 1)
 SET(CYGWIN 1)
 
+SET(CMAKE_EXE_LINKER_FLAGS_INIT "-Wl,--enable-auto-import")
 SET(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS "-shared -Wl,--export-all-symbols -Wl,--enable-auto-import")
 SET(CMAKE_SHARED_MODULE_CREATE_C_FLAGS ${CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS})
 SET(CMAKE_DL_LIBS "-lgdi32" )

bootstrap

@@ -55,6 +55,13 @@ cmake_bootstrap_system_libs=""
 cmake_bootstrap_qt_gui=""
 cmake_bootstrap_qt_qmake=""
 
+# Determine whether this is a Cygwin environment.
+if echo "${cmake_system}" | grep CYGWIN >/dev/null 2>&1; then
+  cmake_system_cygwin=true
+else
+  cmake_system_cygwin=false
+fi
+
 # Determine whether this is a MinGW environment.
 if echo "${cmake_system}" | grep MINGW >/dev/null 2>&1; then
   cmake_system_mingw=true
@@ -598,6 +605,11 @@ cmake_c_flags=${CFLAGS}
 cmake_cxx_flags=${CXXFLAGS}
 cmake_ld_flags=${LDFLAGS}
 
+# Add Cygwin-specific flags
+if ${cmake_system_cygwin}; then
+  cmake_ld_flags="${LDFLAGS} -Wl,--enable-auto-import"
+fi
+
 # Add Carbon framework on Darwin
 if ${cmake_system_darwin}; then
   cmake_ld_flags="${LDFLAGS} -framework Carbon"