Don't strip obsolete tag names from doxygen build errors

[xdje42]

Describe the bug

Issue #2524 is a doxygen build failure due to an obsolete tag.
Debugging this was more work than necessary because the culprit tag was stripped from the error output by this:

# I want to see errors other than:
#  Warning: Tag `MAX_DOT_GRAPH_HEIGHT' at line 248 of file Doxyfile has become obsolete.
#  To avoid this warning please update your configuration file using "doxygen -u"
string(REGEX REPLACE
  "(^|(\r?\n))[^\r\n]*has become obsolete.\r?\n"
  "\\1" doxygen_u_error "${doxygen_u_error}")

The error output is:

CMake Error at /tmp/dynamorio/drmemory/docs/CMake_doxyfile.cmake:256 (message):
  /usr/bin/doxygen -u failed: This tag has been removed.

whereas it could have been:

/usr/bin/doxygen -u failed: warning: Tag 'DOT_TRANSPARENT' at line 243 of file 'Doxyfile' has become obsolete.

           This tag has been removed.

To Reproduce

$ make htmldocs

Versions

• What version of Dr. Memory are you using? git head @bfbc4a118a58a182770d306120cad41b9893c53