[C/C++] CMake: In-tree consumers use different target than external consumers

[ethanmusser]

CMake builds that include libsbp must link against a different target than those that link against an externally-built libsbp.

Description

In-tree consumers must link against sbp.

libsbp/c/src/CMakeLists.txt

Lines 40 to 42 in 0a93888

	swift_add_library(sbp
	SOURCES ${libsbp_SRCS}
	)

Out-of-tree consumers must link against swiftnav::sbp.

libsbp/c/src/CMakeLists.txt

Lines 89 to 93 in 0a93888

	install(
	EXPORT sbp-export
	DESTINATION ${CMAKE_INSTALL_DATADIR}/libsbp/cmake
	NAMESPACE swiftnav::
	)

Proposed Solution

Add a library target swiftnav::sbp as an alias for the sbp target.