feature, add option to build Intel LIBBID as a shared library

[steven-varga]

Add a CMake option to build Intel LIBBID as a shared library instead of always statically linking it.

Motivation:
libdecimal is header-only, but it currently forces LIBBID to be built and installed as libbid_static.a. Some deployment models (e.g., multiple executables or language bindings in the same process) benefit from sharing the LIBBID runtime via a single .so/.dylib.

Proposed behavior:
Introduce a top-level option:

option(libdecimal_BUILD_SHARED_LIBS "Build Intel LIBBID as a shared library" OFF)

• When OFF (default): keep the current behavior — static libbid_static.a.
• When ON: build libbid.so / libbid.dylib / libbid.dll and install it as a runtime library.

Scope and constraints:

• The libdecimal target itself remains INTERFACE/header-only.
• The exported target should remain usable as libdecimal::libdecimal regardless of this option.
• Windows support may require WINDOWS_EXPORT_ALL_SYMBOLS or may be documented as unsupported if LIBBID lacks __declspec(dllexport/dllimport) annotations.
• Existing default builds must be unchanged.

Acceptance criteria:

• Default configure + build + install still produces libbid_static.a and all tests pass.
• cmake -Dlibdecimal_BUILD_SHARED_LIBS=ON ... produces a shared library and installs it.
• A downstream consumer can find_package(libdecimal REQUIRED) and link against libdecimal::libdecimal in both modes.
• No regression in the install layout for the default static build.

[steven-varga]

Implemented in branch 38-feature-build-libbid-shared (commit e1e4a76).

Changes:

• Added top-level option libdecimal_BUILD_SHARED_LIBS (default OFF).
• thirdparty/intel-decimal-float/CMakeLists.txt now builds libbid_static as either:
  • STATIC with output name bid_static → libbid_static.a (default)
  • SHARED with output name bid → libbid.so
• Object libraries get POSITION_INDEPENDENT_CODE ON only in shared mode.
• Target name libdecimal::libbid_static is kept for backward compatibility.

Verified:

• Default static build: 11/11 tests pass, produces libbid_static.a.
• Shared build: cmake -Dlibdecimal_BUILD_SHARED_LIBS=ON ... produces libbid.so, 11/11 tests pass, and ldd confirms executables link against the shared library.
• Generated install script correctly targets libbid.so for shared installs.

Note: full cmake --install verification for the shared library is blocked by the unrelated install bugs being fixed in #37 (public-header glob and LIBBID-header scope). Once #37 lands on `staging#, this branch should be rebased to pick up those fixes and complete install validation.