gh-36415: README: Recommend `configure --config-cache --enable-ccache`
    
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This can save a bit of time for developers, although only a fraction of
our configure tests are cached.

Also removes mention of  `--enable-download-from-upstream-url`, which
has been the default for a while now.

Also reducing some excessive messages from spkg-configure

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- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

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URL: #36415
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe

README.md

@@ -319,12 +319,18 @@ in the Installation Guide.
 
         $ ./configure --help
 
-    A notable option for Sage developers is the following:
-
-    - Use `./configure --enable-download-from-upstream-url` to allow
-      downloading packages from their upstream URL if they cannot (yet) be
-      found on the Sage mirrors. This is useful for trying out ticket branches
-      that make package upgrades.
+    Notable options for Sage developers are the following:
+
+    - Use the option `--config-cache` to have `configure`
+      keep a disk cache of configuration values. This gives a nice speedup
+      when trying out ticket branches that make package upgrades, which
+      involves automatic re-runs of the configuration step.
+
+    - Use the option `--enable-ccache` to have Sage install and use the
+      optional package `ccache`, which is preconfigured to keep a
+      disk cache of object files created from source files. This can give
+      a great speedup when switching between different branches, at the
+      expense of disk space use.
 
 10. Optional, but highly recommended: Set some environment variables to
     customize the build.

build/pkgs/glpk/spkg-configure.m4

@@ -8,18 +8,15 @@ SAGE_SPKG_CONFIGURE([glpk], [
            glpk_ver=`$GLPSOL --version | grep ^GLPSOL | $SED -e 's/GLPSOL.*ver, v//g' 2>> config.log`
            AX_COMPARE_VERSION([$glpk_ver], [ge], [$SAGE_GLPK_MINVER], [
                AC_CHECK_HEADER([glpk.h], [], [sage_spkg_install_glpk=yes])
-               AC_SEARCH_LIBS([glp_config], [glpk],
-                  [AC_MSG_RESULT([yes. Use system's glpk])], [
-                  AC_MSG_RESULT([no. Install glpk])
-                  sage_spkg_install_glpk=yes])dnl end-AC_SEARCH_LIBS
-                  ], [sage_spkg_install_glpk=yes])dnl end-AX_COMPARE_VERSION
+               AC_SEARCH_LIBS([glp_config], [glpk], [], [sage_spkg_install_glpk=yes])dnl end-AC_SEARCH_LIBS
+           ], [sage_spkg_install_glpk=yes])dnl end-AX_COMPARE_VERSION
          ])dnl end-AS_IF
     ])
     m4_popdef([SAGE_GLPK_MINVER])
 ], [], [], [
     AS_IF([test x$sage_spkg_install_glpk = xyes], [
         AC_SUBST(SAGE_GLPK_PREFIX, ['$SAGE_LOCAL'])
-        AC_MSG_RESULT([using Sage's glpk SPKG])], [
+      ], [
         AC_SUBST(SAGE_GLPK_PREFIX, [''])
-        AC_MSG_RESULT([using glpk from the system])])
+      ])
 ])

build/pkgs/mpc/spkg-configure.m4

@@ -7,9 +7,7 @@ SAGE_SPKG_CONFIGURE([mpc], [
 ], [], [], [
     if test x$sage_spkg_install_mpc = xyes; then
         AC_SUBST(SAGE_MPC_PREFIX, ['$SAGE_LOCAL'])
-        AC_MSG_RESULT([using Sage's mpc SPKG])
     else
        AC_SUBST(SAGE_MPC_PREFIX, [''])
-       AC_MSG_RESULT([using mpc library from the system])
     fi
 ])

build/pkgs/mpfr/spkg-configure.m4

@@ -7,9 +7,7 @@ SAGE_SPKG_CONFIGURE([mpfr], [
 ], [], [], [
     if test x$sage_spkg_install_mpfr = xyes; then
         AC_SUBST(SAGE_MPFR_PREFIX, ['$SAGE_LOCAL'])
-        AC_MSG_RESULT([using Sage's mpfr SPKG])
     else
        AC_SUBST(SAGE_MPFR_PREFIX, [''])
-       AC_MSG_RESULT([using mpfr library from the system])
     fi
 ])

build/pkgs/pkgconf/spkg-configure.m4

@@ -4,9 +4,8 @@ SAGE_SPKG_CONFIGURE(
     AS_IF([test -z "$PKG_CONFIG"], [
        sage_spkg_install_pkgconf=yes
        AC_SUBST(SAGE_PKG_CONFIG_PATH, [''])
-       AC_MSG_RESULT([installing pkgconf spkg])], [
+    ], [
 dnl the following as needed as long as Sage creates .pc files during build and/or configure
        AC_SUBST(SAGE_PKG_CONFIG_PATH, ['$SAGE_LOCAL/lib/pkgconfig'])
-       AC_MSG_RESULT([using pkg-config from the system])
     ])
 ])