Merge pull request #109 from tskit-dev/compiler_flag

avoid errors in gcc 8.2.0

Author: jeromekelleher

python/Makefile

@@ -2,7 +2,7 @@
 all: ext3
 
 ext3: _tskitmodule.c 
-	CFLAGS="-std=c99 -Wall -Wextra -Werror -Wno-unused-parameter -Wno-missing-field-initializers" \
+	CFLAGS="-std=c99 -Wall -Wextra -Werror -Wno-unused-parameter -Wno-missing-field-initializers -Wno-cast-function-type" \
 	       python3 setup.py build_ext --inplace
 
 ext2: _tskitmodule.c