finance/libobx: Fix numerous issues

I came to fix only one problem with libofx, but when it failed to pass
stage-QA checks, I found others.

1. CXXFLAGS not propagated (PHB 12.9) due to bug in configure (blanket)
2. DragonFly iconv support (blanket, taken from DPorts)
3. DOXYGEN option was no implemented
   • --disable-doxygen was set rather than DOXYGEN_CONFIGURE_ENABLE
   • DOXYGEN_BUILD_DEPENDS was not set
4. disable doxygen setting, html documentation got installed unconditionally, but this was not reflected in plist

I decided that --disable-doxygen was the correct option because html files
were already provided an installed, so the DOXYGEN option was removed. I
used PORTDOCS to fix the pkg-plist. This port passes stage-QA now.

Approved by: blanket and just-fix-it