android: fix 32bits builds (#1917)

MuPDF would fail to open some ZIPs:

> MuPDF cannot open file.: cannot find end of central directory (7)

Make sure all code is compiled with `-U_FILE_OFFSET_BITS -D_FILE_OFFSET_BITS=32`.

NOTE: the initial `-U_FILE_OFFSET_BITS` is for meson external
projects (cf. mesonbuild/meson#3519).

Makefile.defs

@@ -519,6 +519,7 @@ else ifeq ($(TARGET), android)
 		# NOTE: to be removed when upgrading the minimal supported API level to 24.
 		# (https://android.googlesource.com/platform/bionic/+/master/docs/32-bit-abi.md)
 		export ac_cv_sys_file_offset_bits=32
+		COMPAT_CFLAGS += -U_FILE_OFFSET_BITS -D_FILE_OFFSET_BITS=32
 	endif
 else ifeq ($(TARGET), remarkable)
 	ARM_ARCH:=$(ARMV7_A9_ARCH)