[cmake] install paths not hardwired + use GNUInstallDirs convention

resolves #165

CMakeLists.txt

@@ -60,6 +60,7 @@ include(FeatureSummary)
 include(RedefaultableOption)
 include(CMakePackageConfigHelpers)
 include(AddCustomTargetSubproject)
+include(CMakePushCheckState)
 include(CTest) # defines BUILD_TESTING option
 
 # Configure options
@@ -77,28 +78,27 @@ add_feature_info("TARGET_MAX_INDEX_RANK=${TARGET_MAX_INDEX_RANK}" TRUE "default
 
 set(TARGET_ARCH "${CMAKE_SYSTEM_NAME}-${CMAKE_SYSTEM_PROCESSOR}")
 
-include(CMakePushCheckState)
-include(GNUInstallDirs)
-include(AppendFlags)
-
 ##########################
-# Standard build variables
+# INSTALL variables
 ##########################
-set(BTAS_INSTALL_BINDIR "bin"
+include(GNUInstallDirs)
+set(BTAS_INSTALL_BINDIR "${CMAKE_INSTALL_BINDIR}"
     CACHE PATH "BTAS BIN install directory")
-set(BTAS_INSTALL_INCLUDEDIR "include"
+set(BTAS_INSTALL_INCLUDEDIR "${CMAKE_INSTALL_INCLUDEDIR}"
     CACHE PATH "BTAS INCLUDE install directory")
-set(BTAS_INSTALL_LIBDIR "lib"
+set(BTAS_INSTALL_LIBDIR "${CMAKE_INSTALL_LIBDIR}"
     CACHE PATH "BTAS LIB install directory")
-set(BTAS_INSTALL_SHAREDIR "share/BTAS/${BTAS_MAJOR_VERSION}.${BTAS_MINOR_VERSION}.${BTAS_MICRO_VERSION}"
-    CACHE PATH "BTAS SHARE install directory")
-set(BTAS_INSTALL_DATADIR "${BTAS_INSTALL_SHAREDIR}/data"
+set(BTAS_INSTALL_DATADIR "${CMAKE_INSTALL_DATAROOTDIR}/BTAS/${BTAS_EXT_VERSION}"
     CACHE PATH "BTAS DATA install directory")
-set(BTAS_INSTALL_DOCDIR "${BTAS_INSTALL_SHAREDIR}/doc"
+set(BTAS_INSTALL_DOCDIR "${BTAS_INSTALL_DATADIR}/doc"
     CACHE PATH "BTAS DOC install directory")
-set(BTAS_INSTALL_CMAKEDIR "lib/cmake/BTAS"
+set(BTAS_INSTALL_CMAKEDIR "${CMAKE_INSTALL_LIBDIR}/cmake/BTAS"
     CACHE PATH "BTAS CMAKE install directory")
 
+##########################
+# Standard build variables
+##########################
+include(AppendFlags)
 # Get standard build variables from the environment if they have not already been set
 if(NOT CMAKE_C_FLAGS OR NOT DEFINED CMAKE_C_FLAGS)
   set(CMAKE_C_FLAGS "$ENV{CPPFLAGS}")