The answers you got are (of course) correct. Here are the links to longdouble related documentation:

• INSTALL: Long doubles
• INSTALL: "more bits"
• INSTALL: quadmath
• Glossary: alignbytes
• Glossary: d_Gconvert
• Glossary: d_ilogbl
• Glossary: d_ldbl_dig
• Glossary: d_llrintl
• Glossary: d_long_double_style_ieee
• Glossary: d_long_double_style_ieee_doubledouble
• Glossary: d_long_double_style_ieee_extended
• Glossary: d_long_double_style_ieee_std
• Glossary: d_long_double_style_vax
• Glossary: d_longdbl
• Glossary: d_lrintl
• Glossary: d_modflproto
• Glossary: d_nexttoward
• Glossary: d_PRIeldbl
• Glossary: d_PRIEUldbl
• Glossary: d_PRIfldbl
• Glossary: d_PRIFUldbl
• Glossary: d_PRIgldbl
• Glossary: d_PRIGUldbl
• Glossary: d_SCNfldbl
• Glossary: d_truncl
• Glossary: longdblinfbytes
• Glossary: longdblkind
• Glossary: longdblmantbits
• Glossary: longdblnanbytes
• Glossary: longdblsize
• Glossary: sPRIeldbl
• Glossary: sPRIEUldbl
• Glossary: sPRIfldbl
• Glossary: sPRIFUldbl
• Glossary: sPRIgldbl
• Glossary: sPRIGUldbl
• Glossary: sSCNfldbl
• Glossary: uselongdouble
• Glossary: usemorebits
• Glossary: usequadmath
• config_h.SH: MEM_ALIGNBYTES
• config_h.SH: HAS_LONG_DOUBLE
• config_h.SH: LONG_DOUBLESIZE
• config_h.SH: HAS_LDEXPL
• config_h.SH: LONG_DOUBLEKIND
• config_h.SH: LONG_DOUBLE_STYLE_IEEE
• config_h.SH: LONG_DOUBLE_STYLE_IEEE_DOUBLEDOUBLE
• config_h.SH: LONG_DOUBLE_STYLE_IEEE_EXTENDED
• config_h.SH: LONG_DOUBLE_STYLE_IEEE_STD
• config_h.SH: LONG_DOUBLE_STYLE_VAX
• config_h.SH: HAS_ATOLF
• config_h.SH: HAS_FINITEL
• config_h.SH: HAS_FP_CLASSL
• config_h.SH: HAS_FPCLASSL
• config_h.SH: HAS_FREXPL
• config_h.SH: HAS_ISFINITEL
• config_h.SH: HAS_ISINFL
• config_h.SH: HAS_ISNANL
• config_h.SH: HAS_J0L
• config_h.SH: HAS_LDBL_DIG
• config_h.SH: HAS_LLRINTL
• config_h.SH: HAS_LLROUNDL
• config_h.SH: HAS_LRINTL
• config_h.SH: HAS_LROUNDL
• config_h.SH: HAS_MODFL
• config_h.SH: HAS_NEXTTOWARD
• config_h.SH: HAS_SQRTL
• config_h.SH: HAS_STRTOLD
• config_h.SH: LONGDBLINFBYTES
• config_h.SH: LONGDBLNANBYTES
• config_h.SH: PERL_PRIfldbl
• config_h.SH: PERL_PRIgldbl
• config_h.SH: PERL_PRIeldbl
• config_h.SH: PERL_SCNfldbl
• config_h.SH: LONGDBLMANTBITS
• config_h.SH: NVSIZE
• config_h.SH: USE_LONG_DOUBLE
• config_h.SH: USE_MORE_BITS
• config_h.SH: HAS_STRTOD_L
• config_h.SH: HAS_STRTOLD_L