add compiler warnings checks from LibreSSL

configure.ac

@@ -32,6 +32,8 @@ AC_PROG_LIBTOOL
 LT_INIT
 AC_PROG_YACC
 
+DISABLE_COMPILER_WARNINGS
+
 CHECK_OS_OPTIONS
 
 CHECK_CRYPTO_COMPAT

m4/disable-compiler-warnings.m4

@@ -0,0 +1,29 @@
+AC_DEFUN([DISABLE_COMPILER_WARNINGS], [
+# Clang throws a lot of warnings when it does not understand a flag. Disable
+# this warning for now so other warnings are visible.
+AC_MSG_CHECKING([if compiling with clang])
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [[
+#ifndef __clang__
+	not clang
+#endif
+	]])],
+	[CLANG=yes],
+	[CLANG=no]
+)
+AC_MSG_RESULT([$CLANG])
+AS_IF([test "x$CLANG" = "xyes"], [CLANG_FLAGS=-Qunused-arguments])
+CFLAGS="$CFLAGS $CLANG_FLAGS"
+LDFLAGS="$LDFLAGS $CLANG_FLAGS"
+
+# Removing the dependency on -Wno-pointer-sign should be a goal. These are
+# largely unsigned char */char* mismatches in asn1 functions.
+save_cflags="$CFLAGS"
+CFLAGS=-Wno-pointer-sign
+AC_MSG_CHECKING([whether CC supports -Wno-pointer-sign])
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([])],
+	[AC_MSG_RESULT([yes])]
+	[AM_CFLAGS=-Wno-pointer-sign],
+	[AC_MSG_RESULT([no])]
+)
+CFLAGS="$save_cflags $AM_CFLAGS"
+])