build: define TZDIR for tzcode build

nixos (and probably some other distributives) place zoneinfo directory
not in /usr/share (in /etc for example). TZDIR is set accordingly.
Currently zoneinfo is looked for in /usr/share, disregarding TZDIR env
variable.

This commit adds compile definition for TZDIR if such env variable is
defined. This fixes zoneinfo lookup for nixos.

NO_CHANGELOG=build
NO_DOC=build
NO_TEST=build

src/lib/tzcode/CMakeLists.txt

@@ -13,6 +13,10 @@ set(TZDATA_FULL_TARGET ${TZCODE_SOURCE_DIR}/${TZDATA_TARGET})
 set(TZCODE_SRC ${PROJECT_SOURCE_DIR}/src/lib/tzcode)
 set(TZLUA_SRC ${PROJECT_SOURCE_DIR}/src/lua)
 
+if(DEFINED ENV{TZDIR})
+    add_definitions(-DTZDIR="$ENV{TZDIR}")
+endif()
+
 ExternalProject_Add(${TZCODE_PROJECT}
     EXCLUDE_FROM_ALL 1
     DOWNLOAD_DIR ${TZCODE_BINARY_DIR}