Trac #31552: Update singular to stop /usr/local from leaking into Singular build

On macOS, even when using `gcc -isysroot
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk` (which
disables use of `/usr/local`), Singular's `configure` script sneaks in
`/usr/local/include` and `/usr/local/lib`.

{{{
It was created by singular configure 4.2.0p1+2021-03-13+sage, which was
generated by GNU Autoconf 2.69.  Invocation command line was

  $ ./configure --prefix=/var/tmp/sage-9.3.rc0-cpython-38-darwin/local
--libdir=/var/tmp/sage-9.3.rc0-cpython-38-darwin/local/lib --disable-
maintainer-mode
 --disable-dependency-tracking --exec-
prefix=/var/tmp/sage-9.3.rc0-cpython-38-darwin/local
--bindir=/var/tmp/sage-9.3.rc0-cpython-38-darwin/local/bin --wit
h-ntl=/var/tmp/sage-9.3.rc0-cpython-38-darwin/local --with-
flint=/var/tmp/sage-9.3.rc0-cpython-38-darwin/local --enable-gfanlib
--enable-Singular --enable-
factory --disable-doc --disable-polymake --without-python --without-
pythonmodule --disable-python --disable-python_module --disable-python-
module --disable
-static

PATH: /private/var/tmp/sage-9.3.rc0-cpython-38-darwin/build/bin
PATH: /private/var/tmp/sage-9.3.rc0-cpython-38-darwin/src/bin
PATH: /var/tmp/sage-9.3.rc0-cpython-38-darwin/local/bin
PATH: /private/var/tmp/sage-9.3.rc0-cpython-38-darwin/build/bin
PATH: /private/var/tmp/sage-9.3.rc0-cpython-38-darwin/src/bin
PATH: /var/tmp/sage-9.3.rc0-cpython-38-darwin/local/bin
PATH: /private/var/folders/38/wnh4gf1552g_crsjnv2vmmww0000gp/T/pip-
build-env-dcsf70hx/overlay/bin
PATH: /private/var/folders/38/wnh4gf1552g_crsjnv2vmmww0000gp/T/pip-
build-env-dcsf70hx/normal/bin
PATH: /Users/mkoeppe/s/sage/sage-rebasing/worktree-dist/src/pkgs
/sage_conf-relocatable/.tox/python-macos-10.15-python3.8/bin
PATH: /usr/local/opt/xz/bin
PATH: /usr/local/opt/gpatch/bin
PATH: /usr/bin
PATH: /bin
PATH: /usr/sbin
PATH: /sbin

configure:20714: checking gmp.h usability
configure:20714: gcc -isysroot
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -c -O2 -g
-pipe -fno-common -g0 -O3 -Wno-unused-function -Wno-trigraphs -Wno-
unused-parameter -Wunknown-pragmas -Wno-unused-variable -fomit-frame-
pointer -fwrapv -fvisibility=default -finline-functions -fno-exceptions
-fno-threadsafe-statics -funroll-loops -fno-delete-null-pointer-checks
-Qunused-arguments -pthread -I/usr/local/include  conftest.c >&5
configure:20714: $? = 0
configure:20714: result: yes
}}}
If NTL from homebrew is installed, this can lead to an NTL threading
mismatch:
{{{
    [singular-4.2.0p1+2021-03-13+sage]   ld: illegal thread local
variable reference to regular symbol __ZN3NTL13ErrorCallbackE for
architecture x86_64
    [singular-4.2.0p1+2021-03-13+sage]   clang: error: linker command
failed with exit code 1 (use -v to see invocation)
}}}

PR: Singular/Singular#1066

See also: #31348 `build/pkgs/{mpir,mpfr}/spkg-configure.m4`: Check pkg-
config first

URL: https://trac.sagemath.org/31552
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): François Bissey

Author: Release Manager

build/pkgs/singular/checksums.ini

@@ -1,5 +1,5 @@
 tarball=singular-VERSION.tar.gz
-sha1=eeb0b250e481e710daea45c2f78ccff9b84e6f15
-md5=88c4500c2d6ac3eb9b112111fc9404ad
-cksum=3131197750
-upstream_url=https://trac.sagemath.org/raw-attachment/ticket/25993/singular-VERSION.tar.gz
+sha1=3c8e4d867a050d676f1eaee622694d30c1e00bcc
+md5=edd42d42f3ddaa4f495a2111c72bf0da
+cksum=1198605855
+upstream_url=https://trac.sagemath.org/raw-attachment/ticket/31552/singular-VERSION.tar.gz

build/pkgs/singular/package-version.txt

@@ -1 +1 @@
-4.2.0p1+2021-03-13+sage
+4.2.0p1+2021-03-24+sage-2