How can I replace a given -Ldir1 by -Ldir2, in a linking command line generated by cmake

I have a CMakeLists.txt. I use it for generating a makefile with ccmake. Then upon make, my sources are compiled ok. At link time, the command line produced is essentially

/opt/rh/devtoolset-6/root/usr/bin/c++ myprog.cc.o -o myprog -Ldir3 -L/opt/rh/devtoolset-3/root/usr/lib/gcc/x86_64-redhat-linux/4.9.2 ... -Wl,-rpath,dir4:dir5:/opt/rh/devtoolset-3/root/usr/lib/gcc/x86_64-redhat-linux/4.9.2 ...

The two spots specifying the search path

/opt/rh/devtoolset-3/root/usr/lib/gcc/x86_64-redhat-linux/4.9.2

should actually point to

/opt/rh/devtoolset-6/root/usr/lib/gcc/x86_64-redhat-linux/6.2.1

How can I fix this?

I have devtoolset-3 in my system, but I do not know where this search path is set, or how to change it. I actually expected that to take place automatically after executing

scl-devtoolset-6

(in my .bashrc), the same way the correct version /opt/rh/devtoolset-6/root/usr/bin/c++ is detected without me doing anything else.

Of course, I get many linking time errors due to version mismatches.

The only place where I see the search path set is in line

link_directories(${LIBDIR_MY})

in CMakeLists.txt, and LIBDIR_MY points to dir3, which is correctly added in the linking command line. But I found no place where .../devtoolset-3/... is added.

Possible origins of -L:

link_directories in CMakeLists.txt: checked.
target_link_libraries: Where? What is the expected file name pattern to look for?
link_libraries: Where? What is the expected file name pattern to look for?
CMAKE_LIBRARY_PATH: Checked. It is not set.
A find_package command: See below
Somewhere else?

This How do I add a library path in cmake? does not add to my question.

Update 1:

There was in fact a find_package(mylib) (actually, with a different name) in CMakeLists.txt. Going to the dir of mylib and issuing find . -name “*” -type f -exec grep /opt/rh/devtoolset-3 {} \; there were two files that matched:

build/CMakeCache.txt: two occurrences of devtoolset-3

PETSC_LIBRARIES:STRING=...devtoolset-3...
FIND_PACKAGE_MESSAGE_DETAILS_PETSc:INTERNAL=[...devtoolset-3...][YES][v()]

It appears to me that this stems from file CMake/cmake-modules/FindPETSc.cmake (possibly invoked by line find_package (PETSc REQUIRED) in CMakeLists.txt), which has a line

set (PETSC_LIBRARIES ${PETSC_LIBRARIES_ALL} CACHE STRING "PETSc libraries" FORCE)

and many prior lines

set (PETSC_LIBRARIES_<various terms> ...)

Notes: I do not know where in that file devtoolset-3 is first detected and set.

build/include/summit/mylibConfig.cmake. I still could not track down what made devtoolset-3 appear here.

Solution

I found the culprit. As hinted at in Update 1 of the OP, the sequence is the following:

Line find_package (PETSc REQUIRED) in file (1) CMakeLists.txt forced processing file (2) CMake/cmake-modules/FindPETSc.cmake.
Line^*1 petsc_get_variable (PETSC_EXTERNAL_LIB_BASIC petsc_libs_external) in file (2) CMake/cmake-modules/FindPETSc.cmake sets a local cmake variable petsc_libs_external from the value of the variable PETSC_EXTERNAL_LIB_BASIC read from file (3) ~/petsc-3.8.2/lib/petsc/conf/petscvariables.
PETSC_EXTERNAL_LIB_BASIC is not set explicitly in file (3) ~/petsc-3.8.2/lib/petsc/conf/petscvariables. Line include ~/petsc-3.8.2/arch-linux2-c-debug/lib/petsc/conf/petscvariables forces reading file (4) ~/petsc-3.8.2/arch-linux2-c-debug/lib/petsc/conf/petscvariables.
Line PETSC_EXTERNAL_LIB_BASIC = ... -Wl,-rpath,/opt/rh/devtoolset-3/root/usr/lib/gcc/x86_64-redhat-linux/4.9.2 -L/opt/rh/devtoolset-3/root/usr/lib/gcc/x86_64-redhat-linux/4.9.2 -Wl,-rpath,/opt/rh/devtoolset-3/root/usr/lib64 -L/opt/rh/devtoolset-3/root/usr/lib64 -Wl,-rpath,/opt/rh/devtoolset-3/root/usr/lib -L/opt/rh/devtoolset-3/root/usr/lib ... in file (4) /home/sserebrinsky/petsc-3.8.2/arch-linux2-c-debug/lib/petsc/conf/petscvariables brings the "undesired" directories into the executed command line.

^*1 (petsc_get_variable is a macro defined in FindPETSc.cmake)