Use PCDIR CMake option, if configured

Upstream install the server pkgconfig file into arch-independent directory

Reported to upstream as: https://jira.mariadb.org/browse/MDEV-14340

--- mariadb-10.5.5/support-files/CMakeLists.txt.old	2020-09-30 10:36:08.582490318 +0200

+++ mariadb-10.5.5/support-files/CMakeLists.txt	2020-09-30 10:38:58.079710848 +0200

@@ -91,7 +91,11 @@

   ENDIF()

   CONFIGURE_FILE(mariadb.pc.in ${CMAKE_CURRENT_BINARY_DIR}/mariadb.pc @ONLY)

+IF(INSTALL_PCDIR)

+  INSTALL(FILES ${CMAKE_CURRENT_BINARY_DIR}/mariadb.pc DESTINATION ${INSTALL_PCDIR} COMPONENT Development)

+ELSE()

   INSTALL(FILES ${CMAKE_CURRENT_BINARY_DIR}/mariadb.pc DESTINATION ${INSTALL_LIBDIR}/pkgconfig COMPONENT Development)

+ENDIF()

   INSTALL(FILES mysql.m4 DESTINATION ${INSTALL_SHAREDIR}/aclocal COMPONENT Development)