I have the poco package installed, but alas, sanjuuni will not install.

[Hellscaped]

:: Resolving dependencies...
:: Calculating conflicts...
:: Calculating inner conflicts...

Aur (1) sanjuuni-0.4-1

:: Proceed to review? [Y/n]: y

:: Downloading PKGBUILDs...
PKGBUILDs up to date
nothing new to review
fetching devel info...
==> Making package: sanjuuni 0.4-1 (Tue 18 Jun 2024 12:32:21 AM CDT)
==> Retrieving sources...
-> Found 0.4.tar.gz
==> Validating source files with sha256sums...
0.4.tar.gz ... Passed
==> Making package: sanjuuni 0.4-1 (Tue 18 Jun 2024 12:32:32 AM CDT)
==> Checking runtime dependencies...
==> Checking buildtime dependencies...
==> Retrieving sources...
-> Found 0.4.tar.gz
==> Validating source files with sha256sums...
0.4.tar.gz ... Passed
==> Removing existing $srcdir/ directory...
==> Extracting sources...
-> Extracting 0.4.tar.gz with bsdtar
==> Starting prepare()...
==> Sources are ready.
sanjuuni-0.4-1: parsing pkg list...
==> Making package: sanjuuni 0.4-1 (Tue 18 Jun 2024 12:32:54 AM CDT)
==> Checking runtime dependencies...
==> Checking buildtime dependencies...
==> WARNING: Using existing $srcdir/ tree
==> Removing existing $pkgdir/ directory...
==> Starting build()...
checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether the compiler supports GNU C... yes
checking whether gcc accepts -g... yes
checking for gcc option to enable C11 features... none needed
checking for g++... g++
checking whether the compiler supports GNU C++... yes
checking whether g++ accepts -g... yes
checking for g++ option to enable C++11 features... none needed
checking whether C++ compiler accepts -std=c++11... yes
checking for stdio.h... yes
checking for stdlib.h... yes
checking for string.h... yes
checking for inttypes.h... yes
checking for stdint.h... yes
checking for strings.h... yes
checking for sys/stat.h... yes
checking for sys/types.h... yes
checking for unistd.h... yes
checking for libavformat/avformat.h... yes
checking for libavcodec/avcodec.h... yes
checking for libavdevice/avdevice.h... yes
checking for libswscale/swscale.h... yes
checking for libswresample/swresample.h... yes
checking for zlib.h... yes
checking for Poco/Base64Encoder.h... no
configure: error: Could not find Poco Foundation headers.
==> ERROR: A failure occurred in build().
Aborting...
error: failed to build 'sanjuuni-0.4-1':
error: packages failed to build: sanjuuni-0.4-1

[Commandcracker]

Do you have poco with C headers installed ?
See https://pkgs.org/search/?q=poco-dev