Use GNUInstallDirs with CACHE variables for install destinations (#1152)

Replace hardcoded `set()` with `set(... CACHE STRING ...)` defaulting to
GNUInstallDirs values. This allows Debian/Ubuntu packagers to override
installation directories (e.g. `-DBTCPP_LIB_DESTINATION=lib/x86_64-linux-gnu`)
for multiarch support while preserving backward compatibility.

Fixes #1120

Author: wooksong

CMakeLists.txt

@@ -28,9 +28,11 @@ option(USE_VENDORED_FLATBUFFERS "Use the bundled version of flatbuffers" ON)
 option(USE_VENDORED_MINICORO "Use the bundled version of minicoro" ON)
 option(USE_VENDORED_MINITRACE "Use the bundled version of minitrace" ON)
 
-set(BTCPP_LIB_DESTINATION     lib)
-set(BTCPP_INCLUDE_DESTINATION include)
-set(BTCPP_BIN_DESTINATION     bin)
+include(GNUInstallDirs)
+
+set(BTCPP_LIB_DESTINATION     "${CMAKE_INSTALL_LIBDIR}"     CACHE STRING "Library installation directory")
+set(BTCPP_INCLUDE_DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}" CACHE STRING "Header installation directory")
+set(BTCPP_BIN_DESTINATION     "${CMAKE_INSTALL_BINDIR}"     CACHE STRING "Binary installation directory")
 
 set(BASE_FLAGS "")