Split openmeshcore static/shared MinGW compilation

As with ms compilers, the shared/static compilation depends on export macros.
Anyway this is inconsistent with the behavior for openmeshtools library.

Why did you add this ?
7 jobs for patch-1 in 71 minutes and 19 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #6721
Linux
clang-c++11

00:20:59

passed #6723
Linux
clang-c++98

00:16:44

passed #6726
Linux
cppcheck

00:00:26

passed #6720
Linux
gcc-c++11

00:04:33

passed #6722
Linux
gcc-c++98

00:04:17

passed #6724
Apple
macos-c++11

00:15:44

passed #6725
Apple
macos-c++98

00:08:32