Fix failures on MacOS (#617)

* Fix detection of system ffi

We need ffi_closure_alloc() since commit cf10854 but
don't use ffi_prep_closure() any longer.

* Prepend include path for bundled libffi to make sure it is preferred

When have_header('ffi.h') was successful the system libffi include
path is added to INCFLAGS. But when have_func() fails later on
the bundled libffi which should be used instead.

Using the wrong ffi.h resulted in compiler warnings about undefined
functions and leaded to segfaults on MacOS.

ext/ffi_c/extconf.rb

@@ -11,15 +11,15 @@
   $CFLAGS.gsub!(/[\s+]-std=[^\s]+/, '')
   # solaris 10 needs -c99 for <stdbool.h>
   $CFLAGS << " -std=c99" if RbConfig::CONFIG['host_os'] =~ /solaris(!?2\.11)/
-  
+
   if ENV['RUBY_CC_VERSION'].nil? && (pkg_config("libffi") ||
      have_header("ffi.h") ||
      find_header("ffi.h", "/usr/local/include", "/usr/include/ffi"))
 
-    # We need at least ffi_call and ffi_prep_closure
+    # We need at least ffi_call and ffi_closure_alloc
     libffi_ok = have_library("ffi", "ffi_call", [ "ffi.h" ]) ||
                 have_library("libffi", "ffi_call", [ "ffi.h" ])
-    libffi_ok &&= have_func("ffi_prep_closure")
+    libffi_ok &&= have_func("ffi_closure_alloc")
 
     # Check if the raw api is available.
     $defs << "-DHAVE_RAW_API" if have_func("ffi_raw_call") && have_func("ffi_prep_raw_closure")

ext/ffi_c/libffi.bsd.mk

@@ -3,11 +3,11 @@
 # Makefile for BSD systems
 #
 
-INCFLAGS += -I${LIBFFI_BUILD_DIR}/include
 LOCAL_LIBS += ${LIBFFI} -lpthread
 
 LIBFFI_CFLAGS = ${FFI_MMAP_EXEC} -pthread
 LIBFFI_BUILD_DIR = ${.CURDIR}/libffi-${arch}
+INCFLAGS := -I${LIBFFI_BUILD_DIR}/include -I${INCFLAGS}
 
 .if ${srcdir} == "."
   LIBFFI_SRC_DIR := ${.CURDIR}/libffi

ext/ffi_c/libffi.gnu.mk

@@ -5,11 +5,11 @@
 
 
 # Tack the extra deps onto the autogenerated variables
-INCFLAGS += -I"$(LIBFFI_BUILD_DIR)"/include
 LOCAL_LIBS += $(LIBFFI)
 BUILD_DIR = $(shell pwd)
 LIBFFI_CFLAGS = $(FFI_MMAP_EXEC)
 LIBFFI_BUILD_DIR = $(BUILD_DIR)/libffi-$(arch)
+INCFLAGS := -I"$(LIBFFI_BUILD_DIR)"/include $(INCFLAGS)
 
 ifeq ($(srcdir),.)
   LIBFFI_SRC_DIR := $(shell pwd)/libffi

ext/ffi_c/libffi.vc.mk

@@ -4,7 +4,7 @@
 #
 
 # Tack the extra deps onto the autogenerated variables
-INCFLAGS = $(INCFLAGS) -I$(LIBFFI_BUILD_DIR)/include -I$(LIBFFI_BUILD_DIR)/src/x86
+INCFLAGS = -I$(LIBFFI_BUILD_DIR)/include -I$(LIBFFI_BUILD_DIR)/src/x86 $(INCFLAGS)
 LOCAL_LIBS = $(LOCAL_LIBS) $(LIBFFI)
 BUILD_DIR = $(MAKEDIR)
 LIBFFI_BUILD_DIR = $(BUILD_DIR)/libffi

ext/ffi_c/libffi.vc64.mk

@@ -4,7 +4,7 @@
 #
 
 # Tack the extra deps onto the autogenerated variables
-INCFLAGS = $(INCFLAGS) -I$(LIBFFI_BUILD_DIR)/include -I$(LIBFFI_BUILD_DIR)/src/x86
+INCFLAGS = -I$(LIBFFI_BUILD_DIR)/include -I$(LIBFFI_BUILD_DIR)/src/x86 $(INCFLAGS)
 LOCAL_LIBS = $(LOCAL_LIBS) $(LIBFFI)
 BUILD_DIR = $(MAKEDIR)
 LIBFFI_BUILD_DIR = $(BUILD_DIR)/libffi