Make building libraries static or shared selectable

Default is still to make a shared object because this is the most
versatile target (it can be included in other shared objects as well
as executables).

Note that due to limitations in CMake, either a shared or a static
library can be built, but not both. (This is due to the fact that on
Windows, an import library has the same name as a static library).

configure

@@ -10,6 +10,9 @@ Installation directories:
 Optional Features:
   --disable-FEATURE       do not include FEATURE
   --disable-gxx11check    do not try flag -std=c++11 to enable C++11 features
+  --enable-shared         build a shared library [default=yes]
+  --enable-static         build a static library [default=no]. Note: only one
+                          of the options shared and static may be built.
 
 Optional Packages:
   --with-boost=PATH       use Boost library from a specified location
@@ -117,6 +120,23 @@ while getopts -- ":-:" optchar; do
           esac
           FEATURES="${FEATURES} -DCMAKE_DISABLE_FIND_PACKAGE_${pkgname}=TRUE"
           ;;
+        enable-*)
+          # what kind of library are we building; shared or static?
+          kind=${OPTARG#enable-}
+          case "${kind}" in
+            shared)
+              shared="ON"
+              ;;
+            static)
+              shared="OFF"
+              ;;
+            *)
+              invalid_arg --enable-${kind}
+              exit 1
+              ;;
+          esac
+          FEATURES="${FEATURES} -DBUILD_opm-core_SHARED:BOOL=${shared}"
+          ;;
         *)
           # remove everything *after* the equal sign
 		  arg=${OPTARG%=*}