autoconf: Correctly set Requires.private when compiled with nettle

fixes #16

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>
compat: Add stoken__strcasestr()

Introduce a new src/compat.c to add functions that are unavailable on
non-Linux-glibc platforms.  The first addition is strcasestr(), which
is missing on MinGW.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
autoconf: check for mkstemps

mkstemps is missing on uClibc, and until recently was also missing with
musl-libc. Add an explicit check for this function.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
compat: Add stoken__mkstemps()

mkstemps() is missing on uClibc, so add a local copy (borrowed from
the Bionic / FreeBSD libc).

fixes #18

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
buildsys: Add linker and libtool flags for building Win32 DLLs

This is currently targeted for MinGW only.  References:

http://stackoverflow.com/questions/17487891/libtool-undefined-symbols-not-allowedin-i686-pc-mingw32-shared
http://www.gnu.org/software/libtool/manual/html_node/LT_005fINIT.html

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
library: Use STOKEN_EXPORT to implement Win32 __declspec requirement

On Windows, DLL imports/exports need to be explicitly declared.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
buildsys: Link with -lgmp if using nettle

stc-nettle.c uses gmp functions like mpz_init(), so we should link with
libgmp explicitly.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>