Allow building shared or static library only

[ffontaine]

Patchs retrieved from:

• https://git.buildroot.net/buildroot/tree/package/zmqpp/0001-Allow-building-shared-or-static-library-only.patch
• https://git.buildroot.net/buildroot/tree/package/zmqpp/0002-Install-static-library-for-static-builds.patch

Both patches have been merged in a single one and slightly updated to keep default behavior of building and installing static library (BUILD_STATIC is set to yes by default)

Signed-off-by: Jörg Krause joerg.krause@embedded.rocks Signed-off-by: Gustavo Zacarias gustavo@zacarias.com.ar Signed-off-by: Fabrice Fontaine fontaine.fabrice@gmail.com