[C/C++] CMake: Resolve package configuration install destination mismatch (#1536)

# Description

@swift-nav/algint-team

Fix a mismatch between the configured package configuration file's
install destination and the actual install destination.

Resolves #1533.

# API compatibility

**_Does this change introduce a API compatibility risk?_**

No, the source code is not changed. Only consumers building and then
relocating their package configurations should be affected, and it
should simplify the process without breaking existing workflows.

## API compatibility plan

**_If the above is "Yes", please detail the compatibility (or migration)
plan:_**

N/A

# JIRA Reference

N/A

Author: ethanmusser

c/src/CMakeLists.txt

@@ -92,12 +92,13 @@ install(
   NAMESPACE swiftnav::
 )
 
+set(LIBSBP_CFG_INSTALL_DIR "${CMAKE_INSTALL_DATADIR}/libsbp/cmake")
 configure_package_config_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/libsbpConfig.cmake.in
-  libsbpConfig.cmake
-  INSTALL_DESTINATION ${CMAKE_INSTALL_DATADIR}
+  ${CMAKE_CURRENT_BINARY_DIR}/libsbpConfig.cmake
+  INSTALL_DESTINATION ${LIBSBP_CFG_INSTALL_DIR}
 )
 install(
   FILES "${CMAKE_CURRENT_BINARY_DIR}/libsbpConfig.cmake"
-  DESTINATION ${CMAKE_INSTALL_DATADIR}/libsbp/cmake
+  DESTINATION ${LIBSBP_CFG_INSTALL_DIR}
 )