math/blahtexml: revert/redo previous bogus build "fix" against Clang 16

• Patch the makefile to respect CXXFLAGS so that USE_CXXSTD=c++11 works as intended and remove the whole `USES+=llvm:max=15' abomination
• While here, convert to USES+=iconv, and drop MAKE_ARGS and LDFLAGS as we need to patch that line in the makefile anyway

Fixes: f3eeadc739f9