Modules/Test*.cmake: Use if(DEFINED) to simplify conditions

Replace old hacks of the form 'if("${VAR}" MATCHES "^${VAR}$")'
with the much simpler 'if(NOT DEFINED ${VAR})'.
FindGTK2: Avoid depending on if() to dereference a quoted variable

Explicitly dereference GTK2_${_var}CONFIG_INCLUDE_DIR and
GTK2_${_var}_INCLUDE_DIR when comparing their values.
libarchive: Avoid depending on if() to dereference a quoted variable
cmcurl: Use if(DEFINED) to simplify conditions

Replace old hacks of the form 'if("${VAR}" MATCHES "^${VAR}$")'
with the much simpler 'if(NOT DEFINED ${VAR})'.
FPHSA: Avoid if() dereferencing of quoted variable

Legacy invocations may pass a variable name where "DEFAULT_MSG" belongs.
When comparing FPHSA_FAIL_MESSAGE to "DEFAULT_MSG", use a leading "x" on
both sides to avoid mistaking the value of the message for a variable
name.
Merge topic 'if-sanity' into next

f8933de0 FPHSA: Avoid if() dereferencing of quoted variable
499fe053 cmcurl: Use if(DEFINED) to simplify conditions
cfa3a5e8 libarchive: Avoid depending on if() to dereference a quoted variable
7c369c0b FindGTK2: Avoid depending on if() to dereference a quoted variable
a570216a Modules/Test*.cmake: Use if(DEFINED) to simplify conditions