In Modular X, x11-libs/libXext-1.0.0 needs x11-proto/inputproto in order to compile. It doesn't say so when the compile breaks, but the config.log tells the tale.
Please attach a compilation log and config.log.
Compilation log (such as it is): checking pkg-config is at least version 0.9.0... yes checking for XEXT... configure: error: Package requirements (xproto x11 xextproto xau) were not met. Consider adjusting the PKG_CONFIG_PATH environment variable if you installed software in a non-standard prefix. Alternatively you may set the XEXT_CFLAGS and XEXT_LIBS environment variables to avoid the need to call pkg-config. See the pkg-config man page for more details.
Created attachment 76809 [details] config.log
OK, here's the error so we've got it handy: configure:20258: checking for XEXT configure:20263: $PKG_CONFIG --exists --print-errors "xproto x11 xextproto xau" Package inputproto was not found in the pkg-config search path. Perhaps you should add the directory containing `inputproto.pc' to the PKG_CONFIG_PATH environment variable Package 'inputproto', required by 'X11', not found Fixed by moving inputproto from DEPEND to RDEPEND in libX11.