Compiling with libxml in MacOS Catalina
The problem
When compiling Emacs 28.05, make bootstrap failed with fatal error: 'libxml/tree.h' file not found while autogen.sh and configure were successfully executed.
Troubleshooting
The error message pointed out the culprit, libxml. It is not a part of the MacOS nor the Xcode command line tools now, but I have installed it with homebrew a while ago (because the xml package in R also depends on it, and
I struggled quite a while with it.)
To tell the compiler where to find libxml, I first tried to set LDFLAGS="-L /usr/local/opt/libxml2/lib" and CPPFLAGS="-I /usr/local/opt/libxml2/include". I made two mistakes here: First, I edit src/Makefile instead of the one in the emacs directory. However, it does not matter after all because make bootstrap attempts to compile in a clean state and the old Makefile that I edited is removed before compiling.
Google lead me to
this very help post in which db48x pointed out that if ./configure knows where the package configuration is, it will set the flags accordingly. We can set export PKG_CONFIG_PATH="/usr/local/opt/libxml2/lib/pkgconfig:$PKG_CONFIG_PATH to tell ./configure where libxml2 is.
Curiously, when I set PKG_CONFIG_PATH in R (Sys.setenv("PKG_CONFIG_PATH"="/usr/local/opt/libxml2/lib/pkgconfig:$PKG_CONFIG_PATH")), installing XML from source still fails with the following error message:
You are trying to use a version 2.* edition of libxml but an incompatible library. The header files and library seem to be mismatched. If you have specified LIBXML_INCDIR, make certain to also specify an appropriate LIBXML_LIBDIR if the libxml2 library is not in the default directories.
On the other hand, the solution in the previous post still works: Sys.setenv(XML_CONFIG = "/usr/local/opt/libxml2/bin/xml2-config"). I tried both the Xcode copy (in /usr/local/opt) and the homebrew copy (in /usr/local/Cellar), and both worked. Note that if you update the homebrew copy regularly, you’ll need to change the path because the path contains the version number (for example, /usr/local/Cellar/libxml2/2.9.10_1/).
