Make: Use relative install path for C sources

The install() command for C sources was using a hardcoded absolute
path `/usr/src/`, which ignores CMAKE_INSTALL_PREFIX and breaks
builds on non-standard environments like Termux or when using
custom installation prefixes.

Relates to ashvardanian/NumKong#287

Author: ashvardanian

CMakeLists.txt

@@ -861,5 +861,5 @@ if (STRINGZILLA_INSTALL)
     endif ()
 
     install(DIRECTORY ${STRINGZILLA_INCLUDE_BUILD_DIR} DESTINATION ${STRINGZILLA_INCLUDE_INSTALL_DIR})
-    install(DIRECTORY ./c/ DESTINATION /usr/src/${PROJECT_NAME}/)
+    install(DIRECTORY c/ DESTINATION ${CMAKE_INSTALL_DATADIR}/${PROJECT_NAME}/src)
 endif ()