Gdb Build Fix, master branch (2019.11.06.)

Fixed the Gdb configuration to use Python correctly. During the move to use a skeleton configure.sh.in file, one setting (for LDFLAGS) was lost. :-( This prevented the build with Python 3 to succeed.

The build with Python 2.7 (at least on CentOS 7) did succeed, since the OS provides Python 2.7 as well. Masking the shortcoming of the configuration.

This is to fix the issue that showed up in the nightly against LCG_96python3, as discussed in ATLINFR-3221.

AE Build SUCCESS
Build logfiles are available at Jenkins [AE-MERGE-REQUEST-CC7 #109]

merged

mentioned in commit 4499368b