gcc: Turn FORTRAN into a weak assignment

This ensures that it can be turned on from a distro layer
config metadata e.g. local.conf without forcing overrides

currently we have do something like

FORTRAN:forcevariable = ",fortran"
RUNTIMETARGET:append:pn-gcc-runtime = " libquadmath "
TOOLCHAIN_TARGET_TASK:append = " gfortran"

to enabled fortran support from distro conf file.

After this change we can do it with

FORTRAN = ",fortran"
RUNTIMETARGET:append:pn-gcc-runtime = " libquadmath "
TOOLCHAIN_TARGET_TASK:append = " gfortran"

(From OE-Core rev: 50abae55eda0115d073bbc6552f2fcb4ecbb7949)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/gcc/gcc-15.1.inc

@@ -79,7 +79,7 @@ S = "${TMPDIR}/work-shared/gcc-${PV}-${PR}/${SOURCEDIR}"
 B = "${WORKDIR}/gcc-${PV}/build.${HOST_SYS}.${TARGET_SYS}"
 
 # Language Overrides
-FORTRAN = ""
+FORTRAN ?= ""
 
 SSP ?= "--disable-libssp"
 SSP:mingw32 = "--enable-libssp"

meta/recipes-devtools/gcc/gcc-configure-common.inc

@@ -6,7 +6,7 @@ require gcc-shared-source.inc
 # These can be overridden by the version specific .inc file.
 
 # gcc 3.x expects 'f77', 4.0 expects 'f95', 4.1 and 4.2 expect 'fortran'
-FORTRAN ?= ",fortran"
+FORTRAN ??= ",fortran"
 LANGUAGES ?= "c,c++${FORTRAN}"
 
 EXTRA_OECONF_BASE ?= ""