[enhancement](build-be) Support customizing extra compile flags

[adonis0147]

Proposed changes

Add an environment variable EXTRA_CXX_FLAGS to customize the extra compile flags.

Problem summary

In our daily work, someones may want to change the compile flags to satisfy their requirements. This pr introduces a way to customize extra compile flags.

E.g.

Change the optimization level:

1. Change it once.

BUILD_TYPE=ASAN EXTRA_CXX_FLAGS=-O3 ./build.sh --be

2. Change it during login.

export EXTRA_CXX_FLAGS=-O3
BUILD_TYPE=ASAN ./build.sh --be

3. Store it in file.

echo EXTRA_CXX_FLAGS=-O3 >> custom_env.sh
BUILD_TYPE=ASAN ./build.sh --be

Reference:

If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

Optimize-Options

Checklist(Required)

1. Does it affect the original behavior:
   • Yes
   • No
   • I don't know
2. Has unit tests been added:
   • Yes
   • No
   • No Need
3. Has document been added or modified:
   • Yes
   • No
   • No Need
4. Does it need to update dependencies:
   • Yes
   • No
5. Are there any changes that cannot be rolled back:
   • Yes (If Yes, please explain WHY)
   • No

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[jacktengg]

LGTM

[yiguolei]

LGTM