Fix Mac OS X iconv library linking failure

Add a path to awesomeConfig.cmake for searching an iconv library and the
corresponding header file automatically if the build system is on Mac OS
X which needs to pass the header and library paths explicitly to the
compiler to build Awesome successfully.

Closes https://github.com/awesomeWM/awesome/pull/440.

awesomeConfig.cmake

@@ -155,6 +155,43 @@ if(NOT AWESOME_REQUIRED_FOUND OR NOT AWESOME_COMMON_REQUIRED_FOUND)
     message(FATAL_ERROR)
 endif()
 
+# On Mac OS X, the executable of Awesome has to be linked against libiconv
+# explicitly.  Unfortunately, libiconv doesn't have its pkg-config file,
+# and CMake doesn't provide a module for looking up the library.  Thus, we
+# have to do everything for ourselves...
+if(APPLE)
+    if(NOT DEFINED AWESOME_ICONV_SEARCH_PATHS)
+        set(AWESOME_ICONV_SEARCH_PATHS /opt/local /opt /usr/local /usr)
+    endif()
+
+    if(NOT DEFINED AWESOME_ICONV_INCLUDE_DIR)
+        find_path(AWESOME_ICONV_INCLUDE_DIR
+                  iconv.h
+                  PATHS ${AWESOME_ICONV_SEARCH_PATHS}
+                  PATH_SUFFIXES include
+                  NO_CMAKE_SYSTEM_PATH)
+    endif()
+
+    if(NOT DEFINED AWESOME_ICONV_LIBRARY_PATH)
+        get_filename_component(AWESOME_ICONV_BASE_DIRECTORY ${AWESOME_ICONV_INCLUDE_DIR} DIRECTORY)
+        find_library(AWESOME_ICONV_LIBRARY_PATH
+                     NAMES iconv
+                     HINTS ${AWESOME_ICONV_BASE_DIRECTORY}
+                     PATH_SUFFIXES lib)
+    endif()
+
+    if(NOT DEFINED AWESOME_ICONV_LIBRARY_PATH)
+        message(FATAL_ERROR "Looking for iconv library - not found.")
+    else()
+        message(STATUS "Looking for iconv library - found: ${AWESOME_ICONV_LIBRARY_PATH}")
+    endif()
+
+    set(AWESOME_REQUIRED_LDFLAGS
+        ${AWESOME_REQUIRED_LDFLAGS} ${AWESOME_ICONV_LIBRARY_PATH})
+    set(AWESOME_REQUIRED_INCLUDE_DIRS
+        ${AWESOME_REQUIRED_INCLUDE_DIRS} ${AWESOME_ICONV_INCLUDE_DIR})
+endif(APPLE)
+
 macro(a_find_library variable library)
     find_library(${variable} ${library})
     if(NOT ${variable})