I modified the documentation for BSD users to force them to use gmake instead of make, added some tips about this issue and reverted my patch.
'$' -> '$$' is apparently now the default substitution to use for all new gmake release (and four dollars will fail). :/

On BSD distributions, if you use make instead of gmake, U++ compilation will fail

Build requires: gmake gtk2 freetype2 libnotify llvm39 (clang++)

% which clang++
/usr/bin/clang++
% clang++ --version | head -1
FreeBSD clang version 3.4.1 (tags/RELEASE_34/dot1-final 208032) 20140512

I read it in 10720 revision. But why did you use word "will" (instead of "may") in the following snippet of text?:
<<On BSD distributions, if you use make instead of gmake, U++ compilation will fail>>

For example, this is not an issue for uppsrc/umk (which also exported for current tarball), where there are no files with "$" characters for dependencies and current revision builds fine with BSD make. This is an issue for uppsrc/ide or other projects, where files with "$" character(s) may be used.

Also, I noticed about build requirements:

Build requires: gmake gtk2 freetype2 libnotify llvm39 (clang++)

Maybe this is true for some *BSD operating systems without Clang compiler in base, but Clang 3.4.1 on FreeBSD 10.3 is capable to build uppsrc/ide and uppsrc/umk projects in tarball (was tested for 10703 revision):

% which clang++
/usr/bin/clang++
% clang++ --version | head -1
FreeBSD clang version 3.4.1 (tags/RELEASE_34/dot1-final 208032) 20140512

Installation of devel/llvm39 FreeBSD port is not mandatory for building of uppsrc/ide and uppsrc/umk (current revisions), at least on FreeBSD 10.3 amd64. But even if build/install it, the clang++39 (wrapper script; as part of devel/llvm39 FreeBSD port) may be used, because clang++ may be installed to different directory (which may be not exposed in (common) PATH, because other clang++ may exist in /usr/bin).

What package should I install? llvm39 is what TrueOS tells me to install by default if clang++ is not installed.

% uname -sr
FreeBSD 10.3-RELEASE-p15
% which cc clang c++ clang++
/usr/bin/cc
/usr/bin/clang
/usr/bin/c++
/usr/bin/clang++
% cd /usr/bin && sha256 cc clang c++ clang++
SHA256 (cc) = 2f86f05d3bb79bfcb9be965df7c89e3066c39b7b119796e79e69664c92c53ce0
SHA256 (clang) = 2f86f05d3bb79bfcb9be965df7c89e3066c39b7b119796e79e69664c92c53ce0
SHA256 (c++) = 2f86f05d3bb79bfcb9be965df7c89e3066c39b7b119796e79e69664c92c53ce0
SHA256 (clang++) = 2f86f05d3bb79bfcb9be965df7c89e3066c39b7b119796e79e69664c92c53ce0
% c++ --version | head -1
FreeBSD clang version 3.4.1 (tags/RELEASE_34/dot1-final 208032) 20140512

It's part of the core system, great. Which package does clang++ belong?
"pkg_info -W /usr/bin/clang++" can tell I guess? If I add this to the documentation, it should work for everyone using BSD clones.

% make -V CC -V CXX -V CPP
cc
c++
cpp

Ok, so if I understand you correctly, now upp build scripts (Makefiles, domake, doinstall, upp-devel.spec, ...) are stable enough.

% fetch http://www.ultimatepp.org/downloads/upp-x11-src-10724.tar.gz
<...>
% sha256 upp-x11-src-10724.tar.gz 
SHA256 (upp-x11-src-10724.tar.gz) = 2cec4d158dcc2af851d3b0072ca0a2e7d7ff485da80be1fd4f1c1ba1b0849b4b
% tar -xf upp-x11-src-10724.tar.gz
% cd upp-x11-src-10724
% make CXX=c++
<...>
% ls umk theide
theide	umk
% ./umk
Usage: umk assembly main_package build_method -options [+flags] [output]

Examples: umk examples Bombs GCC -ab +GUI,SHARED ~/bombs
          umk examples,uppsrc Bombs ~/GCC.bm -rv +GUI,SHARED ~/bin

See http://www.ultimatepp.org/app$ide$umk$en-us.html for details
% ./theide --help
Usage: theide assembly package
       theide assembly package build_method [-[a][b][e][r][s][S][v][1][2][m][d][M][l][x][X][Hn]] [+FLAG[,FLAG]...] [out]
       theide -f [file..]
       theide [file..] // autodetection mode

Note:
- Even if you don't force c++ (with CXX=c++), domake will search in your path for g++, than clang++ and finally c++.

% which g++ | xargs pkg which -o
/usr/local/bin/g++ was installed by package lang/gcc
% pkg info -r lang/gcc
gcc-4.9.4:
	kBuild-0.1.9998_6