glxinfo.h:23:10: fatal error: GL/glx.h: No such file or directory

Gentoo 64-bit, driver 396.24

make[1]: Entering directory '/tmp/nvidia-settings/src'
Package gtk+-2.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `gtk+-2.0.pc'
to the PKG_CONFIG_PATH environment variable
No package 'gtk+-2.0' found
Package gtk+-2.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `gtk+-2.0.pc'
to the PKG_CONFIG_PATH environment variable
No package 'gtk+-2.0' found
   CC           command-line.c
In file included from command-line.c:32:0:
glxinfo.h:23:10: fatal error: GL/glx.h: No such file or directory
 #include <GL/glx.h>
          ^~~~~~~~~~
compilation terminated.
make[1]: *** [Makefile:313: _out/Linux_x86_64/command-line.o] Error 1
make[1]: Leaving directory '/tmp/nvidia-settings/src'
make: *** [Makefile:23: all] Error 2

And unfortunately, ordinary nvidia-settings bundled with nvidia-drivers pulls in a lot of various messy stuff...

Start with the reported build output, command-line.c, glxinfo.h, and the Makefile target that compiles _out/Linux_x86_64/command-line.o. Check the pkg-config dependency lookup and the availability of GL/glx.h; done means the project builds successfully on the reported Gentoo 64-bit setup.