libgit2: update to 0.21.0 with thread safe support

It seems to fail due to the http library. Might be that I need to update the http one...

In file included from C:/msys64/home/NICE/MINGW-packages/mingw-w64-libgit2/src/libgit2-0.21.0/deps/regex/regex.c:82:0:
C:/msys64/home/NICE/MINGW-packages/mingw-w64-libgit2/src/libgit2-0.21.0/deps/regex/regcomp.c: In function 'parse_dup_op':
C:/msys64/home/NICE/MINGW-packages/mingw-w64-libgit2/src/libgit2-0.21.0/deps/regex/regcomp.c:2553:39: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
     postorder (elem, mark_opt_subexp, (void *) (long) elem->token.opr.idx);
                                       ^
C:/msys64/home/NICE/MINGW-packages/mingw-w64-libgit2/src/libgit2-0.21.0/deps/regex/regcomp.c: In function 'mark_opt_subexp':
C:/msys64/home/NICE/MINGW-packages/mingw-w64-libgit2/src/libgit2-0.21.0/deps/regex/regcomp.c:3782:19: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
   int idx = (int) (long) extra;
                   ^
Linking C shared library libgit2.dll
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.1/../../../../lib\libhttp_parser.a(http_parser.o):(.text+0x58e): undefined reference to `__assert_func'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.1/../../../../lib\libhttp_parser.a(http_parser.o):(.text+0x795): undefined reference to `__assert_func'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.1/../../../../lib\libhttp_parser.a(http_parser.o):(.text+0x10fb): undefined reference to `__assert_func'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.1/../../../../lib\libhttp_parser.a(http_parser.o):(.text+0x16c5): undefined reference to `__assert_func'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.1/../../../../lib\libhttp_parser.a(http_parser.o):(.text+0x1876): undefined reference to `__assert_func'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.1/../../../../lib\libhttp_parser.a(http_parser.o):(.text+0x1b10): more undefined references to `__assert_func' follow
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.1/../../../../x86_64-w64-mingw32/bin/ld.exe: C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.1/../../../../lib\libhttp_parser.a(http_parser.o): bad reloc address 0x0 in section `.rdata'
collect2.exe: error: ld returned 1 exit status
CMakeFiles/git2.dir/build.make:3164: recipe for target 'libgit2.dll' failed
make[2]: *** [libgit2.dll] Error 1
CMakeFiles/Makefile2:60: recipe for target 'CMakeFiles/git2.dir/all' failed
make[1]: *** [CMakeFiles/git2.dir/all] Error 2
Makefile:127: recipe for target 'all' failed
make: *** [all] Error 2
==> ERROR: A failure occurred in build().
    Aborting...