Fix mingw32 building

Improved cygwin and ming32 support, related to https://github.com/radare/radare2/issues/1099

Still this error though (while building radare2 on top of it, looks like not cygwin/ming32 related:

```
make static
make[4]: Entering directory `/d/Work/radare/radare2/shlr/sdb/src'
make libsdb.a
make[5]: Entering directory `/d/Work/radare/radare2/shlr/sdb/src'
make[5]: `libsdb.a' is up to date.
make[5]: Leaving directory `/d/Work/radare/radare2/shlr/sdb/src'
make[4]: Leaving directory `/d/Work/radare/radare2/shlr/sdb/src'
make shared
make[4]: Entering directory `/d/Work/radare/radare2/shlr/sdb/src'
make libsdb.dll
make[5]: Entering directory `/d/Work/radare/radare2/shlr/sdb/src'
gcc -c  -g  -o cdb._o cdb.c
gcc -c  -g  -o buffer._o buffer.c
gcc -c  -g  -o cdb_make._o cdb_make.c
gcc -c  -g  -o ls._o ls.c
gcc -c  -g  -o ht._o ht.c
gcc -c  -g  -o sdb._o sdb.c
gcc -c  -g  -o num._o num.c
gcc -c  -g  -o base64._o base64.c
gcc -c  -g  -o json._o json.c
gcc -c  -g  -o ns._o ns.c
gcc -c  -g  -o lock._o lock.c
gcc -c  -g  -o util._o util.c
gcc -c  -g  -o disk._o disk.c
gcc -c  -g  -o query._o query.c
gcc -c  -g  -o array._o array.c
gcc -c  -g  -o fmt._o fmt.c
gcc -shared -shared -Wl,-soname,libsdb.so. -o libsdb.dll cdb._o buffer._o cdb_ma
e._o ls._o ht._o sdb._o num._o base64._o json._o ns._o lock._o util._o disk._o
query._o array._o fmt._o
make[5]: Leaving directory `/d/Work/radare/radare2/shlr/sdb/src'
make[4]: Leaving directory `/d/Work/radare/radare2/shlr/sdb/src'
make[3]: Leaving directory `/d/Work/radare/radare2/shlr/sdb/src'
make -C memcache
make[3]: Entering directory `/d/Work/radare/radare2/shlr/sdb/memcache'
gcc -c  -g  -o cmds.o cmds.c
cmds.c:3:17: fatal error: sdb.h: No such file or directory
compilation terminated.
make[3]: *** [cmds.o] Error 1
make[3]: Leaving directory `/d/Work/radare/radare2/shlr/sdb/memcache'
make[2]: *** [all] Error 2
make[2]: Leaving directory `/d/Work/radare/radare2/shlr/sdb'
make[1]: *** [sdb/sdb] Error 2
make[1]: Leaving directory `/d/Work/radare/radare2/shlr'
make: *** [all] Error 2
```
Cygwin *and* mingw?