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7.3 Special Characters in Output Variables
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Many output variables are intended to be evaluated both by ‘make’ and by
the shell.  Some characters are expanded differently in these two
contexts, so to avoid confusion these variables' values should not
contain any of the following characters:

     " # $ & ' ( ) * ; < > ? [ \ ^ ` |

   Also, these variables' values should neither contain newlines, nor
start with ‘~’, nor contain white space or ‘:’ immediately followed by
‘~’.  The values can contain nonempty sequences of white space
characters like tabs and spaces, but each such sequence might
arbitrarily be replaced by a single space during substitution.

   These restrictions apply both to the values that ‘configure’
computes, and to the values set directly by the user.  For example, the
following invocations of ‘configure’ are problematic, since they attempt
to use special characters within ‘CPPFLAGS’ and white space within
‘$(srcdir)’:

     CPPFLAGS='-DOUCH="&\"#$*?"' '../My Source/ouch-1.0/configure'

     '../My Source/ouch-1.0/configure' CPPFLAGS='-DOUCH="&\"#$*?"'