Googled this issue, tried all the recommended solutions, and I found nothing helpful.
When I run this command,
btool -file build-x64.xml
Everything compiles correctly, but the linker completely fails to find all external libraries:
Make error line 459: LINK problem: c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lMagick++-6.Q16 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lMagickCore-6.Q16 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lMagickWand-6.Q16 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -latk-1.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -latkmm-1.6 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lcairo c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lcairomm-1.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lcdr-0.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lexif c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lfontconfig c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lfreetype c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lgc c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lgdk-win32-2.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lgdk_pixbuf-2.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lgdkmm-2.4 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lgio-2.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lgiomm-2.4 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lglib-2.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lglibmm-2.4 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lgobject-2.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lgsl c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lgslcblas c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lgthread-2.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lgtk-win32-2.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lgtkmm-2.4 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lintl c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -ljpeg c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -llcms2 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lpango-1.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lpangocairo-1.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lpangoft2-1.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lpangomm-1.4 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lpangowin32-1.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lpng c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lpoppler c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lpoppler-glib c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lpopt c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lsigc-2.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -ltiff c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lvisio-0.0 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lwpd-0.9 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lwpd-stream-0.9 c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lwpg-0.2
The thought occurred to me that the issue might also affect the regular 32-bit builds as well. It didn't. They still compile fine. Johan, Partha, help!