Build from the installed deltachat version

This cleans up the build somewhat:
- Move time_t declaration to _build.py
- Uses Python's distutils to invoke the "correct" compiler.  Or at
  least does not make this code responsible to picking a compiler
- Uses the compiler to find the installed header file, builds
  against this installed version.  Since linking will also be
  against this installed version this seems to make some sense.

This seems somewhat ugly though.

python/src/deltachat/_build.py

@@ -1,23 +1,35 @@
-import subprocess
+import distutils.ccompiler
+import distutils.sysconfig
 import tempfile
 
 import cffi
 
 
-ffibuilder = cffi.FFI()
-ffibuilder.set_source(
+def ffibuilder():
+    builder = cffi.FFI()
+    builder.set_source(
         'deltachat.capi',
         """
             #include <deltachat/mrmailbox.h>
         """,
         libraries=['deltachat'],
-)
-with tempfile.NamedTemporaryFile(mode='r') as fp:
-    proc = subprocess.run(['gcc', '-E', '-o', fp.name, '-DPY_CFFI=1',
-                           '../src/mrmailbox.h'])
-    proc.check_returncode()
-    ffibuilder.cdef(fp.read())
+    )
+    builder.cdef("""
+        typedef int... time_t;
+    """)
+    cc = distutils.ccompiler.new_compiler(force=True)
+    distutils.sysconfig.customize_compiler(cc)
+    with tempfile.NamedTemporaryFile(mode='w', suffix='.h') as src_fp:
+        src_fp.write('#include <deltachat/mrmailbox.h>')
+        src_fp.flush()
+        with tempfile.NamedTemporaryFile(mode='r') as dst_fp:
+            cc.preprocess(source=src_fp.name,
+                          output_file=dst_fp.name,
+                          macros=[('PY_CFFI', '1')])
+            builder.cdef(dst_fp.read())
+    return builder
 
 
 if __name__ == '__main__':
-    ffibuilder.compile(verbose=True)
+    builder = ffibuilder()
+    builder.compile(verbose=True)

src/mrmailbox.h

@@ -150,18 +150,15 @@ extern "C" {
 #ifndef PY_CFFI
 #include <pthread.h>
 #include <libetpan/libetpan.h> /* defines uint16_t */
+#endif
+
 #include "mrarray.h"
-#endif
 #include "mrchatlist.h"
-#ifndef PY_CFFI
 #include "mrchat.h"
-#endif
 #include "mrmsg.h"
 #include "mrcontact.h"
-#ifndef PY_CFFI
 #include "mrlot.h"
 #include "mrevent.h"
-#endif
 
 
 /**

src/mrmsg.h

@@ -26,10 +26,6 @@
 extern "C" {
 #endif
 
-#ifdef PY_CFFI
-typedef int... time_t;
-#endif
-
 
 typedef struct _mrmailbox mrmailbox_t;