configure: errors if static/shared is not specified

[GitMensch]

Rechecked with GMP: if those options are not explicit specified GMP 6.1.2 automatically enables static library, disables shared.
GMP also gives a summary of the build options, both parts look reasonable to me so I suggest to do the same.

configure: summary of build options:

  Version:           GNU MP 6.1.2
  Host type:         i686-pc-mingw32
  ABI:               32
  Install prefix:    /usr/local
  Compiler:          occ --nologo
  Static libraries:  yes
  Shared libraries:  no

[wbhart]

It's weird to me that the chose static as the default. But there seems to be no reason to not do the same as them.