use variable SAGE_GIAC_ENABLED to control giac feature

build/pkgs/giac/spkg-configure.m4

@@ -22,6 +22,7 @@ SAGE_SPKG_CONFIGURE([giac], [
         AC_SEARCH_LIBS([ConvertUTF16toUTF8], [giac], [
         ], [sage_spkg_install_giac=yes])
        ], [sage_spkg_install_giac=yes])
+       AC_SUBST(SAGE_ENABLE_giac)
        m4_popdef([GIAC_MIN_VERSION])
        m4_popdef([GIAC_MAX_VERSION])
     ])

pkgs/sage-conf/_sage_conf/_conf.py.in

@@ -34,6 +34,8 @@ SAGE_NAUTY_BINS_PREFIX = "@SAGE_NAUTY_BINS_PREFIX@"
 
 SAGE_ECMBIN = "@SAGE_ECMBIN@"
 
+SAGE_GIAC_ENABLED = "@SAGE_ENABLE_giac@"
+
 # Names or paths of the 4ti2 executables
 FOURTITWO_HILBERT = "@FOURTITWO_HILBERT@"
 FOURTITWO_MARKOV = "@FOURTITWO_MARKOV@"

src/sage/env.py

@@ -224,6 +224,7 @@ def var(key: str, *fallbacks: Optional[str], force: bool = False) -> Optional[st
 KENZO_FAS = var("KENZO_FAS")
 SAGE_NAUTY_BINS_PREFIX = var("SAGE_NAUTY_BINS_PREFIX", "")
 SAGE_ECMBIN = var("SAGE_ECMBIN", "ecm")
+SAGE_GIAC_ENABLED = var("SAGE_GIAC_ENABLED", "yes")
 RUBIKS_BINS_PREFIX = var("RUBIKS_BINS_PREFIX", "")
 FOURTITWO_HILBERT = var("FOURTITWO_HILBERT")
 FOURTITWO_MARKOV = var("FOURTITWO_MARKOV")

src/sage/features/giac.py

@@ -4,6 +4,7 @@
 """
 
 from . import Executable, FeatureTestResult
+from sage.env import SAGE_GIAC_ENABLED
 
 class Giac(Executable):
     r"""
@@ -23,7 +24,11 @@ def __init__(self):
             sage: isinstance(Giac(), Giac)
             True
         """
-        Executable.__init__(self, 'giac', executable='giac',
+        if SAGE_GIAC_ENABLED == "no":
+            giac_exe = 'fofobar42barfoo'
+        else:
+            giac_exe = 'giac'
+        Executable.__init__(self, 'giac', executable=giac_exe,
                             spkg='giac', type='optional')
 
 def all_features():