Revert "upgrade to gcc 15"

This reverts commit 3e5789afa6.
Because we may have compatibility problems with gcc 15.

configure/os/CONFIG.Common.RHEL8-x86_64

@@ -4,9 +4,8 @@ include $(CONFIG)/os/CONFIG.Common.linux-x86_64
 BUILD_CLASS = HOST
 
 TOOLSET_LOCATION = /opt/rh
-TOOLSET = gcc-toolset-15
+TOOLSET = gcc-toolset-12
 STD_CXXFLAGS = -std=c++20
-STD_CFLAGS = -std=c17
 
 # Fix bug in gcc-toolset-11 calling the old assembler
 ifneq ($(filter %-11,$(TOOLSET)),)

configure/os/CONFIG.Common.RHEL9-x86_64

@@ -4,9 +4,8 @@ include $(CONFIG)/os/CONFIG.Common.linux-x86_64
 BUILD_CLASS = HOST
 
 TOOLSET_LOCATION = /opt/rh
-TOOLSET = gcc-toolset-15
+TOOLSET = gcc-toolset-12
 STD_CXXFLAGS = -std=c++20
-STD_CFLAGS = -std=c17
 
 # Fix bug in gcc-toolset-11 calling the old assembler
 ifneq ($(filter %-11,$(TOOLSET)),)

configure/os/CONFIG_SITE.Common.linuxCommon

@@ -12,7 +12,6 @@ COMMANDLINE_LIBRARY = READLINE
 
 USR_CXXFLAGS += $(STD_CXXFLAGS)
 STD_CXXFLAGS = -std=c++11
-USR_CFLAGS += $(STD_CFLAGS)
 
 TOOLSET_DIR      = $(TOOLSET:%=$(TOOLSET_LOCATION)/%/root)
 GNU_DIR = $(SYSROOT)$(TOOLSET_DIR)

epics-base.spec

@@ -53,10 +53,10 @@ BuildRequires:  wine
 Summary:    Minimal stuff needed to build EPICS host apps
 Requires:   %{name} = %{version}-%{release}
 
-# This EPICS installation uses the toolset 15 compiler
+# This EPICS installation uses the toolset 12 compiler
 %if 0%{?rhel} >= 8
-Requires:       gcc-toolset-15-gcc-c++
-BuildRequires:  gcc-toolset-15-gcc-c++
+Requires:       gcc-toolset-12-gcc-c++
+BuildRequires:  gcc-toolset-12-gcc-c++
 %endif
 
 # The perl stuff we need