xpdf: add include and link flags required to build

I'm not sure how (or whether) this ever worked without this change.

Without the include tweak, compile error was:

clang: warning: argument unused during compilation: '-/opt/X11/include/freetype2'
In file included from SplashFTFont.cc:15:
/opt/X11/include/ft2build.h:56:10: fatal error: 'freetype/config/ftheader.h' file not found
#include <freetype/config/ftheader.h>
         ^

Without the linker tweak, link error was:

c++ -I/opt/X11/include -I/opt/X11/include/freetype2 -DHAVE_CONFIG_H -I.. -I./../goo -I./../fofi -I./../splash -I.            -o xpdf Annot.o Array.o BuiltinFont.o BuiltinFontTables.o Catalog.o CharCodeToUnicode.o CMap.o CoreOutputDev.o Decrypt.o Dict.o Error.o FontEncodingTables.o Function.o Gfx.o GfxFont.o GfxState.o GlobalParams.o JArithmeticDecoder.o JBIG2Stream.o JPXStream.o Lexer.o Link.o NameToCharCode.o Object.o OptionalContent.o Outline.o OutputDev.o Page.o Parser.o PDFCore.o PDFDoc.o PDFDocEncoding.o PreScanOutputDev.o PSOutputDev.o PSTokenizer.o SecurityHandler.o SplashOutputDev.o Stream.o TextOutputDev.o UnicodeMap.o UnicodeTypeTable.o XPDFApp.o XPDFCore.o XPDFTree.o XPDFViewer.o XpdfPluginAPI.o XRef.o xpdf.o -L../goo -lGoo -L../splash -lsplash  -lfreetype -lXm -lXt -lXp -lXext -lXpm  -lSM -lICE  -lX11  -L../fofi -lfofi -L../goo -lGoo -lm
ld: library not found for -lXt