gh-35737: build/pkgs/maxima: Fix parallel build
    
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### 📚 Description

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Fix a mistake in a maxima Makefile.
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maxima build fails non-deterministically in parallel builds.
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- Fixes
#35619 (comment)

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URL: #35737
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik

Author: Release Manager

build/pkgs/maxima/spkg-install.in

@@ -20,6 +20,10 @@ cd src/
 # Use newer version of config.guess and config.sub (see Trac #19734)
 cp "$SAGE_ROOT"/config/config.* .
 
+# Patch out bad 'multiple targets' rule
+# https://github.com/sagemath/sage/pull/35619#issuecomment-1567351409
+sed -i.bak 's/^maxima_singlepage.html //' doc/info/Makefile.in
+
 # Note that maxima configure checks for git and, if it finds it, uses
 # versions information from the repo. See #15529. We disable this with
 # git_found=false