Work around -std=c++11 MinGW compiler problem

This bug was already fixed 5 months ago, but AFAICT MinGW.org hasn't
released it yet.  MinGW-w64 is unaffected, as is the MinGW compiler
packaged with Cygwin.

Makefile

@@ -29,6 +29,9 @@ UNIX_ADAPTER_EXE ?= console.exe
 ifeq "$(wildcard config.mk)" ""
     $(error config.mk does not exist.  Please run ./configure)
 endif
+
+MINGW_OPTIMIZATION_FLAGS ?= -O2
+
 include config.mk
 
 COMMON_CXXFLAGS += \
@@ -45,7 +48,7 @@ UNIX_CXXFLAGS += \
 
 MINGW_CXXFLAGS += \
 	$(COMMON_CXXFLAGS) \
-	-O2 \
+	$(MINGW_OPTIMIZATION_FLAGS) \
 	-std=c++11
 
 MINGW_LDFLAGS += -static -static-libgcc -static-libstdc++

configure

@@ -148,6 +148,11 @@ echo MINGW_CXX=$MINGW_CXX >> config.mk
 
 if test $IS_MSYS1 = 1; then
     echo UNIX_CXXFLAGS += -DWINPTY_TARGET_MSYS1 >> config.mk
+    # The MSYS1 MinGW compiler has a bug that prevents inclusion of algorithm
+    # and math.h in C++11 mode.  The workaround is to disable optimizations.
+    # It was fixed on 2015-07-31, but as of 2016-01-05, the fix hasn't been
+    # released.  See http://ehc.ac/p/mingw/bugs/2250/.
+    echo MINGW_OPTIMIZATION_FLAGS = >> config.mk
 fi
 
 # Figure out how to embed build info (e.g. git commit) into the binary.